Anemone and Dominic of Eureka Seven. The dangerous drugs she uses to pilot her mech take her Domestic AbuseTsundere type behavior way beyond the usual fare. Sure, it can be funny at first to watch her beat the stuffing out of him, but as you gradually realize the depths of his devotion, her cruel indifference or abject violence becomes incredibly painful to watch. On Anemone's side, when she finally realizes how much he means to her, but believes it's all too late, she delivers a gut-wrenching soliloquy, convinced that she doesn't deserve to live. Amazingly, it all manages to end well.
Roy/Riza in Fullmetal Alchemist. Even aside from the whole burning her back thing, they have been a massive source of pain for one another. Most of the traumatic events they have gone through would most likely never have taken place if they hadn't met each other (Roy wouldn't even be a State Alchemist, she wouldn't have joined the army and been sent to Ishval and he wouldn't have annihilated so many people; she has made it clear she doesn't like being in the army or killing people, but she won't quit as long as he needs her). They're practically the direct (albeit non-intentional) cause of everything in the other's life that makes them suffer, and still they're the very thing that makes the other want to go on living as stated outright in chapter 95.
Riza has her throat cut to force Roy to open the gate. Riza signals with a glare that not only would she beat him to death if he committed human transmutation, but help was on the way so it was pointless. It was nice of Arakawa to not force Roy to choose between the two options, though when you think about the fact that he was seriously considering damning Amestris (and, possibly-eventually the whole world) for saving Riza, and couldn't even come to the decision to not do human transmutation till it seemed she would be saved you get a little worried at the lengths Roy would go to to keep his Lieutenant by his side.
Pick practically ANY hero in ANY Gundam series. If you don't believe me...
Gundam ZZ: Judau Ashta loses Chara Soon, a broken young woman he didn't romantically love but still quite cared about, mere seconds after making her promise that she wouldn't die.
Victory Gundam : Even though nearly every couple in this series doesn't end well, the main character's crush on Katejina ends up very badly since she becomes an enemy AND in love with the resident Char Clone too!
Roybea Roy, the resident Chivalrous Pervert of Gundam X fame is well aware of this, its the reason why he chases away most of his lovers before he aligns with the Freeden crew. This doesn't stop him from falling in with a female member of a La Résistance, who then dies in his arms.
Ennil El happens to fall for The Hero Garrod, who, wanting nothing to do with her, aims a gun in her face; she becomes a Stalker with a Crush who follows the Freeden wanting revenge.
Gundam SEED: Kira Yamato was manipulated by his first girlfriend, Flay, and later saw her die in front of him. Also Murrue Ramius, whose heart goes on. Twice. At least one of them returned in the following series.
Not content to ruin just one couple, Gundam 00 has destroyed several of them. Feldt lost her first crush before she even had the chance of admitting anything to him, and ditto for Tieria who was in love with the same man that Feldt loved; Christina and Lichty were already dying when they conceded that they had feelings for each other; Saji's relationship with Louise is to put it very nicely "on the rocks"; Allelujah and Soma's has also hit some rough ground with the death of Sergei Smirnov; Lyle, the twin brother of the guy loved by Feldt and Tieria, just saw his lover Anew get blown to pieces by Setsuna before his very eyes; Sumeragi's lover Emilio was already dead prior to the series beginning and then what relation she may have had with Billy got torn up when Setsuna opened his mouth about her being part of CB
Flit Asuno from Gundam AGE fell for Yurin L'Ciel, only to see her get manipulated and later brutally killed by Desil. He becomes so enraged and psychologically broken that he not only refuses to acknowledge the UE as fellow human beings, but also starts to see himself as a savior... for the corrupt and genocidal Earth Federal Forces.
Gundam Iron-Blooded Orphans: On Gjallarhorn's side, Carta had unrequited feelings for [McGillis] except the latter just sent her to her death. On Tekkadan-Turbines' side, Lafter slowly had feelings for Akihiro who is completely oblivious about it. Unfortunately, she got shot to death by an assassin under Jasley who just want to get even with Tekkadan. Shino is also unable to recognize Yamagi's feeling for him and then, he gets killed. Even the happy couple, Naze and Amida, are not spared from this particularly in Episode 40 where after Amida's Mobile Suit got shot down, Naze pulls a kamikaze run on the enemy's ship so he can be with her in death. In the finale, Mikazuki dies and doesn't live to see his son with Atra and Kudelia.
Gundam Thunderbolt: On the Feddies' side, Io's girlfriend and commanding officer, Claudia, got too stressed out when she sends her men to die in the battlefield that she took up drugs to forget everything. Then, her XO shoots her for being a horrible leader. And then, it turns out that she survives but is brainwashed by a group of Buddhist fanatics. On Zeon's side, Dr. Karla lost her boyfriend after he got killed by Io but she slowly moved on and became close to Daryl. She even stood by his side when he lost his remaining limbs. They didn't die in the end but Karla went insane after she just watched her friends getting vaporized and got grabbed by a RX-77-2 Guncannon. Poor Daryl can't do anything about it as he hold her in his arms, watching her laugh like a child.
Among those who got off (relatively) easy are:
The 08th MS Team: Shiro Amada only loses a leg, but he keeps Aina and they live in peace.
Gundam 0080: Christina Mackenzie is completely unaware that she killed Bernie.
G Gundam: Domon Kasshu almost lost Rain to the same Devil Gundam that caused the deaths of many others, including his beloved older brother. But he managed to save her and get her back... Anguished Declaration of Love. Also, [[spoiler:Allenby escapes the fate of most Brainwashed and Crazy women because the person who fights her is Rain, who has a Gundam equipped for precision and is a doctor. Rain brings her down without deadly force and defuses her, leaving her with just a brief hospital stay.
Gundam 00 (again) Saji and Louise got back together; Alle and Soma survived their Battle Couple stint and went Walking the Earth; Sumeragi talked Billy outta his funk and they kinda reconciled; Patrick survived too and he and Kati got married.
Gundam Build Fighters: As expected of the most light-hearted Gundam series yet, the romance (confined mostly to Ship Tease) results in far more comedy than angst. Reiji and Aila face the most obstacles, but like almost everyone in the series, they get a happy ending.
Gundam Iron-Blooded Orphans: Merribit and Yukinojo survived after the destruction of Tekkadan. In the epilogue, they ended up having children.
Then, pick any hero in any Gundam-inspired series.
Gendo behaves like he does because his wife's death broke him. He was afraid of other people and Yui was the only person who reached out to him. Without her he did not know how to love other people, and he sent Shinji away because he felt he was unworthy of being a parent and was afraid of hurting him.
In the latter half of the series, it becomes increasingly Shinji and Asuka harbors feelings for each other. But due to their respective psychological issues (which turns out to make them Not So Different from each other), neither of them can fully understand these feelings nor figure out how to express them to each other in any kind of constructive manner. The end result that their mutual misunderstandings of each other breeds resentment and bitterness between them.
One of the main messages of the series (with the addendum "...but you need it.")
Nagisa Kaworu is pretty much the person who is the kindest to Shinji in the whole series. He is nice to Shinji and loves him unconditionally without asking anything back, and is the only one who ever told Shinji that he loves him. And Shinji does reciprocate the feelings, which just makes everything sadder when it's revealed that Kaworu is also an Angel, by default an enemy of humanity that Shinji has to defeat. And Kaworu asks Shinji to kill him because he wants him and other human to survive. Shinji does so, which leads him to his final and greatest emotional breakdown.
Code Geass: Suzaku witnesses his big love Princess Euphemia fatally shot by his arch-nemesis, after which she effectively dies in his arms (Pietà Plagiarism galore), then some fifteen episodes later Lelouch, the said "arch-nemesis", experiences Shirley, a girl he always cared for (well, perhaps not romantically but it was definitely headed that way) dying on him in a very similar fashion... at the hands of his Gay Option of sorts, a Yandere boy who later commits suicide to save Lelouch's own life..
Ayumi from Honey and Clover has a bad case of unrequited love for Takumi, who sees her more like his sister. A lot of the first season of the anime is devoted to Ayumi's suffering because of this. Takumi in turn has a hard time with his feelings for (older) Rika.
In One Piece, "Pirate Empress" Boa Hancock of the Seven Warlords of the Sea has the power to turn anyone who's so much as attracted to her to stone. And then when she herself falls in love, she might die by it.
This is actually subverted when Hancock falls in love, given that she has decided to help Luffy rather than bottle her feelings within her, as that was the cause of death for two empresses before her, and almost killed the elder/ex-ex-ex empress herself. Except that even though her feelings are sincere and intense and based on how he once defeated the Tenryuubito aka those who enslaved and tortured Hancock and her sisters as young girls, it's almost depressinglyapparent that she doesn't really have a chance with Luffy. Though, in all fairness, she has as much chance as any other woman with Luffy, the ultimate Idiot Hero with a degree in Oblivious to Love.
It's also painfully obvious Camie the mermaid has a pretty serious thing for Hatchan, but...well...same reasons as above.
Sanji literally rides this trope like a horse, not only are the bruises Nami leaves on him heart shaped, but tanking brutal beatings so a woman will be unharmed or a wedding dress won't get his blood on it, is all pain Sanji will accept. Even just glancing at Boa Hancock will turn Sanji to stone even when she wasn't even using her powers. Sanji has more tragic examples of this in Whole Cake Island as he must give up on his crew, so that they and his Father figure Zeff will survive. Even being forced to beat down Luffy and letting a heart broken Nami slap him in the face and farewell him... further heart brake ensured Sanji when Pudding the girl he was in a Arranged Marriage with, had planned to kill him during the ceremony.
Lithuania in Axis Powers Hetalia gets all his fingers broken by his crush, the eccentric Yandere Belarus. He doesn't seem to mind. Russia, who is both Lithuania's boss and Bela's older brother (and more of a Yandere than her), has a very twisted, rather one-sided relationship with Lithuania himself, showing his affections through stalking and abuse.
Also, Prussia might have feelings for either North Italy or Hungary. Either way, he's screwed because of their respective romantic ties to Germany (Prussia's younger brother, who is also heavily implied to have feelings for Italy and to be the Holy Roman Empire, Italy's childhood love interest.) and Austria (Hungary's ex-husband).
And if there's the other option of him possibly having feelings for Austria... yup, still a loss either way.
Hajime and Chiaki: Despite her best efforts, Chiaki is unable to convince Hajime that he is worthy of her, contributing to him disappearing to sign up for the Kamukura Project. She then spends an entire year waiting for him until she finally meets him as Izuru, who personally states he doesn't remember her. He then watches her die, triggering some level of too-late recollection as he cries, and holds onto her hairpin in grief and memory.
Kyosuke and Chisa: Kyosuke losing Chisa drives him insane. Then for added fun it's shown she was tortured into Despair during Side:Despair and has been spending the time between Side:Despair and Side:Future corrupting him into the man he is currently.
Makoto and Kyoko: Kyoko willingly sacrifices herself, refusing to betray Makoto again, and leaving him heartbroken.
Ruruka and Izayoi: Despite Izayoi's loyalty, Ruruka murders him so he can't betray her in the future. Izayoi still responds that he loves her, leaving Ruruka full of regret and self-loathing.
Adding insult to injury, unrequited and platonic love are also used to the most hurtful degrees, such as Junko using Juzo's unrequited love for Kyosuke as a way to manipulate him and using the fact that Chiaki is the most loved person in the 77th Class to throw the rest of the class into despair.
In Gantz, Kei goes through severe depression multiple times due to his loves dying on him. The first time is when Kishimoto dies (although she never did return his feelings), and subsequent times of him becoming really depressed are caused by his girlfriend Tae (who died but got revived). And since the series is still going on, there's still no guarantee that they'll be able to be together. A Downer Ending is quite possible...
Ako Izumi of Mahou Sensei Negima!. She has a ton of self-esteem issues, and is described as something of a doormat. Then she meets Nagi, he reassures her of her importance, and she falls in love with him. Even after getting Trapped in Another World and sold into slavery, she's able to hang on because she knows that he's there for her. Too bad that "Nagi" is her ten-year-old teacher Negi magically aged-up. Naturally, she's utterly crushed when she finds out. And then, she gets better, sort of; she still has a thing for 'Nagi'. Which may be transferring to Negi now.
Sagara Sousuke from Full Metal Panic! towards Kaname. His whole Heroic BSoD in TSR was caused by being torn away from her after he developed an attachment and feelings for her. As Gauron angrily notices, ever since falling in love, he seems to have a lot more conflicting feelings and doubts.
And speaking of Gauron, he seems to have lost some of his edge and become even more messed up since falling for Sousuke... after all, if he didn't care about Sousuke, he wouldn't have been so very angry in the first place...
In regards to Gauron, this is actually brought up and symbolically implied in the novels (though apparently parts of it were lost in the fan translation). Members of Amalgam actually covertly bring up to Sousuke how Gauron was overtaken by "cancer," all while mentioning that he "really really liked Sousuke"... and according to the original wording, a parallel is implied of Kaname being Sousuke's "Gan" (cancer), and Sousuke being Gauron's "Gan". Any possible double meaning was lost to Sousuke. In addition, reading about the overall attitude of Gauron's colleagues when they talk to Sousuke, it seems like it's a well known fact to them that Gauron was way too obsessed with Sousuke.
RahXephon: Hey Megumi. What's that, your crush already has a girlfriend who is your best friend? Ouch. Now what about that other guy? Sorry, your sister Haruka already had the key to his heart long ago. Oh, and the other other guy? Turned evil because of the death of the girl he loves.
Jigen from Lupin III. His luck with women runs from bad to worse, with them either betraying him, dying, or both.
Must have also hurt Kittan badly when he only had the confidence (and partly because of it) to confess to Yoko before performing an EPICHeroic Sacrifice to spare her from doing the same...
And there's Viral, who is immortal and can't reproduce... and whose greatest dream is to have a wife and child. *sniff*
Kenshin Himura and Tomoe Yukishiro from Rurouni Kenshin. When he was in his Extreme Doormat phase, she was the one that made him start feeling anything at all again. Apparently it was first caring, then love, and finally awful grief. He has his fair share of Love Hurts moments with his second girlfriend Kaoru too, but his story with Tomoe steals the show.
It's also a whole deal for Tomoe herself. She lost her Victorious Childhood Friend Akira to Kenshin's sword, went to Kyoto to find him and have revenge, but once she knew Kenshin better she fell for him. And she died. Horribly and tragically.
In Magic Knight Rayearth Hikaru and Lantis had it pretty bad. She killed his brother without knowing the true story behind his actions, then spent the entire second season caught between falling in love with Lantis and feeling heartbroken because he probably hated her for killing his only family. Lantis didn't have it any easier. Though he didn't hold her accountable and wasn't averse to loving her after she unknowingly revealed her feelings for him, it all went downhill when she was chosen as a candidate for the Pillar... thus threatening to repeat the tragic romance of his elder brother and the former Pillar Emeraude. Hikaru managed to destroy the Pillar system later on but still was forced to go back to her world after saving Cephiro in the end.
(Almost) everybody in Itoshiki-sensei's class loves their Zetsubou-sensei. Too bad every single one of them is in some way severely unhinged.
Tails and Cosmo from Sonic X were the cutest and sweetest couple ever, but just at the series ends, Cosmo must fuse herself with the Big Bad, Dark Oak Planet to weaken it, so Tails (of all people, to boot) could fire his ship's supercharged cannon to destroy it and save the universe, killing Cosmo and crushing his heart in the process. To make it even worse, Sonic and Shadow's attempts to revive Cosmo are unsuccessful, leaving a completely broken Tails lying on the floor, crying his soul out. Not a total failure though; they managed to get a seed that they imply will grow back into Cosmo with all her memories intact.
This series is rife with this, whether romantic or not. There's Hinata, who wanted to be loved by her clan but was abused and even disowned by them until the Chunin Exams arc in Part I; Gaara, who wanted to be loved but was hated by everyone until he befriended Naruto; Konan, whose Love Interest is none other than Pain; Jiraiya, who was Tsunade's Unlucky Childhood Friend; Kakashi, who learned what friendship and love were when he lost his best friend Obito and then was forced to kill Rin, who loved him and which he promised Obito to protect; Obito, who actually survived and witnessed all this; etc.
This is the lesson learned in Apollo's Song, where a young sociopath with a pathological hatred of love is condemned by the goddess Athena to live through a series of tragic love stories.
In Maiden Rose, the main characters' relationship is the reason their lives are so miserable and screwed up. Their relationship is also exactly why they're both willing to put up with all the misery as they couldn't stand living without each other.
One of Hinagiku's fears in Hayate the Combat Butler is based on this, and it's implied that her fear of heights extends from this. She's afraid of loving someone because her parents left and her older sister has turned to drinking after making sure that Hina doesn't have to deal with the debt and has foster parents, thus she's afraid of loving anyone again, in an effort to keep from getting hurt by them. She may have even turned to her Tsundere personality because of it. Her admitting (to Ayumu) that she does love Hayate has been shown as an effort because of these fears.
Puella Magi Madoka Magica. Sayaka Miki, who becomes a magical girl so that her crush's permanent injuries heal (despite being warned that it won't work the way she wants to), loses said crush to her friend because she is essentially a zombie as result of being a magical girl leading her to feel that she's unlovable (and thus when said friend does give her the chance to confess first to the boy, she cannot take it.) She becomes increasingly self-destructive ending with her turning into a witch, the creatures that magical girls are tasked to fight. In the end when the universe is reset to give all magical girls a happier life Sayaka still ends up dying. Word of God says that, in any continuity where she becomes a Magical Girl, she'll be damned to this and can't be saved.
And that's not even counting the Yuri Genre pairings... Kyoko's care for Sayaka accomplishes nothing except angsting and Kyoko's unnecessary Heroic Sacrifice to destroy Witch!Sakaya. Homura's attempts to save Madoka only end up making both of their lives worse, and even after Madoka hits the Reset Button, they end up indefinitely locked in separate dimensions.
Ranma ½: While many of the romantic hi-jinks are Played for Comedy, it is obvious that all the girls in Ranma's harem are in love with him. Shampoo is probably the most blatant and forward with her feelings for Ranma, concocting multiple schemes to win his heart while brushing off the similarly strong if not stronger affections that Mousse has for her. Ukyo strives to actually be seen as a "woman" by Ranma, while Konatsu probably wishes for Ukyo to see him as an actual man. Kodachi is difficult to place as she is crazy but she does seem to generally have honest feelings for Ranma. She claims that she knows that he doesn't feel the same way but is determined to keep trying. And that brings us to the Akane. Akane is in love with Ranma. There are just a couple of things that stand in the way of their romance actually going anywhere.
Akane also goes through this for years by having a crush on Dr. Tofu, a man who's in love with her sister, Kasumi. It isn't until she's 16 that she finally moves on and develops a crush on Ranma.
Akane: You can stop your feeble attempts to cheer me up now. I've got a broken heart, remember? Ranma: Well, then... Akane: Then what? Ranma: Then you gotta realize that these things happen.
The plot, point, story and character motivation of Minami Ozaki's Zetsuai 1989. Period. Even the bystanders get traumatized by the events.
In chapter 40 of Rosario + Vampire, it is revealed that succubi can die from heartbreak, which seems to be happening to poor Kurumu.
GE - Good Ending has this all over the place with all the girls, but specially Utsumi, the main male protagonist.
There's loads of this in D.N.Angel. For example, there's Daisuke's unreturned feelings for Risa who has a crush on Dark who he sees kissing Risa at one point breaking his heart. Also, occurs in a tragic example between Satoshi and Daisuke in the manga.
Harry MacDoogal from Outlaw Star. You've got to feel for him because he has feelings for Melfina and she doesn't share the same feelings for him and he does everything he can to make her love him only to fail and get curb stomped by a massive douchebag with the ability to break his body parts like a bendy straw.
Revolutionary Girl Utena is rife with this. Juri angsts and has much heartbreak over her feelings for her friend Shiori who loves to torment her but is secretly in love with her.
There's tension between Akio, Anthy, and Utena. Utena sleeps with Akio and falls in love with Anthy and finds out that both her love interests are sleeping together and they're siblings. Utena and Anthy fit the Star-Crossed Lovers trope too in that they both have to endure a world of hurt in order to be together, with Akio himself being the main cause of it. There's also a heartbreaking scene in the anime where Anthy regretfully betrays Utena by stabbing her through from behind by a sword. Also, both Anthy and Akio follow that up by whispering cruel words to Utena about how she's a girl and therefore can't become a noble prince.
Touga is in love with Utena who doesn't show she returns those feelings until right before his death scene.
Nanami has a incestuous yandere crush on Touga and is heartbroken when she witnesses him getting intimate with her followers and other girls and when Touga tosses her aside and tries to rape her in Akios car. He also cruelly manipulates her to his own ends and it's topped off with Nanami giving a tearful Anguished Declaration of Love to him after learning they are not blood related.
The Guts, Griffith, Casca Love Triangle in Berserk. Griffith has two love interests in Guts and Casca and has a fall back plan to marry Casca and have kids should his initial plans fall through. However, Guts and Casca become an item and talk of leaving Griffith behind who was crippled by torture. He snaps and tries to kill himself and ultimately ends up sacrificing everyone but guts and Casca and then Casca is raped in front of Guts by the only other man she loved. Currently, because of the rape Casca reverted to an infantile state and is incapable of returning the affection the love starved and broken Guts desires.
If this wasn't tragic enough, when Farnese and Schierke delve into Casca's mind. They discover beneath her infantile state, Casca still loves Guts but her desires are not the same as his and she sees his equally loyal devotion as a fruitless endeavor. Casca in her mind sees herself as broken shell being dragged around by Guts who also chained to her, worst of all her Guts despite all his care cannot be part of her recovery as the surface-layer of still fears him.
Farnese suffers too since she also is love with Guts (despite being engaged to another man). But of course Guts has given his heart and soul to Casca and Farnese seeing Guts continuously damage his himself to protect her when Casca doesn't even understand, causes Farnese to lash out at her in bitterness and jealousy. Farnese regrets her anger at Casca (whose she supposed looking after), her jealousy gets taken Up to Eleven in Fantasia Arc where the witches tell Guts Casca can be cured which makes Guts incredibly happy and Farnese anxious. Farnese especially gets hit hard when sees the love Casca and Guts had in the former's mind.
Her half brother Serpico reveals, protecting a helpless Casca is the sole purpose and meaning in Farnese's life. Meaning if that were changed, Farnese would be redundant to Guts.
In Katekyō Hitman Reborn! Tsuna has an unreturned crush on Kyouko and Gokudera is implied to have feelings for Tsuna. His soft spot for Tsuna results in many, many touching woobie moments for him. The fact that he's willing to (and many times does) go through hell and high water for Tsuna who just thinks of him as a friend makes many people just want to give him a big hug.
Chaos from Heaven's Lost Property is an unusual spin on this. She believes that pain is love and as a severe Love Freak she seeks to spread her "love" to everyone.
Jeremy in A Cruel God Reigns. His girlfriend Vivi breaks up with him after she suspects that he cheated on her (although he was forced into having sex with Greg so he wouldn't break off his engagement with Jeremy's mother) and he spends the next several books not only reeling at Greg's abuse, but mourning the loss of his beloved girlfriend. Poor guy.
This trope also applies to Ian and Nadia in later books; Ian, because of his confusion about falling in love with Jeremy despite the latter's slight case of Being Tortured Makes You Evil and Nadia because of her heartbreak over losing Ian to Jeremy.
Can also be applied to Sandra when Greg breaks off their engagement because Jeremy refuses him. Also tied in with Sandra's Driven to Suicide and Bungled Suicide.
Happens to Bird's Nest a couple of times in Copernicus Breathing, once after he thinks Leo has died in his juggling accident (which then leads to a Freak Out ) and then again when he confuses Michel Bohringer with his dead little brother Michel (twice) (again, Incest Subtext. )
Sorata also feels this way after seeing those two together. Unlike Misaki, he does interrupt them, but fortunately for him Jin had no intentions of entering the hotel with Mashiro, and set this up knowing both he and Mashiro were being followed by him.
Happens to Nanami in Kamisama Kiss when she confesses her love to Tomoe. See Tomoe is Nanami's familiar and a Little Bit BeastlyKitsune while Nanami is a teenage human girl and Tomoe does not approve of Interspecies Romance. So he immediately tries to force her to take back her confession which works out about well as you can expect.
Rick Hunter/Hikaru Ichijyo has it pretty bad in Robotech and the original Super Dimension Fortress Macross. For most of the series, he's in love with LynnMinmei/Minmay. She, on the other hand, has her own issues with her Idol Singer career and is very Oblivious to Love. Of course, Rick/Hikaru himself isn't blameless, as he pretty much ignores the feelings of the much more mature CO Lisa Hayes/Misa Hayase until it's almost too late. And Lisa/Misa has her own Dark and Troubled Past that includes a lost fiancé among other very complicated issues.
Jellal Fernandez and Erza Scarlet of Fairy Tail have this hard. They were slaves together as children in the Tower of Heaven and were deadset on one day escaping and living their lives together. Then, their plan was discovered, and Erza was dragged away to be punished. Jellal manages to save her, but is punished with unimaginably cruel torture that, coupled with Ultear's brainwashing, drove him completely insane. Erza rescues him only to have him drive her out of the Tower when she refuses to join him in his madness, and she spends the next eight years dreading the day they'll meet again. Eventually they do, and she's forced to leave him to die after their confrontation. A bit of time after that, he's revived with no memories and completely back to his real personality, but is arrested by the authorities for his previous actions right after they reconcile. Shortly afterward, she is put into a coma for seven years, and when they finally meet again, he pushes her away because he feels he does not deserve her love any longer, though he can't quite keep himself from following her around. Their whole relationship is built on this trope.
Zeref. His love for his little brother Natsu and his desire to bring him back to life resulted in him getting cursed to kill those around him the more he values life. That same curse killed the girl he loved, Mavis. In short: love kills, at least for him.
In CLANNAD when Tomoya's wife dies tragically in childbirth, Tomoya feels that he should never have met Nagisa in the first place, because of the horrible pain her death is causing him.
K's Fushimi Saruhiko has an Unrequited Love for Yata Misaki that is apparently so strong that it drove him insane. Because Misaki had found new friends and a new family with HOMRA and started to drift away from him he lost himself in jealousy and heartbreak, believing that making Misaki hate him was the only way to get his attention. The Manga Days of Blue shows very clearly that he still suffers badly over being away.
In Umi Monogatari, this is Oshima and Kanon's situation in a nutshell.
Kanon realizes that it was her own insecurity that made her feel like Kojima was going to dump her, when in fact he had liked her all along and never intended to dump her. At the same time, Kojima confesses to Marin that he had liked Kanon from the start, but felt she was purposely separating herself from him, and so they grew distant. Kanon realizes that she never told anyone she liked them out loud even once.
Oshima initially seems to be your classic Alpha Bitch, but she has heartbreaking hidden depths. She tells Kanon that even when she was going out with Kojima, all he ever talked about was Kanon. She feels she'll never measure up to Kanon in his eyes, and that it hurts to like someone that much knowing they'll never feel the same way about you.
Tsukiyama from Tokyo Ghoul has an obsession towards Kaneki and is said to "love Kaneki above all other things". This is to the point where he breaks down into a sobbing mess when he tries to keep Kaneki from going on a suicide mission who rejects him and ends up in an Angst Coma continuing into the sequel.
Further complicating things is how Tsukiyama comes out of his Angst Coma after he learns from Kanae that Kaneki is still alive but is now Sasaki and doesn't remember his past life as Kaneki and the time he spent with Tsukiyama.
In Chapter 43, Eto confronts Kanae and gives him a Breaking Speech centered around his conflicted feelings about Tsukiyama and Sasaki. She tells him that he's actually afraid of being alone, and his "loyalty" is actually possessiveness over his master. According to her, he enjoyed Tsukiyama being helpless and dependent on him, and found true joy not in helping Tsukiyama reunite with Sasaki......but in tormenting Sasaki and making him suffer. She goes even further, stating that Tsukiyama will never return his feelings. Having broken Kanae down, she suggests that they should break the ones that won't return their love.
In the above encounter, the word Eto uses to describe Kanae's feelings is "Aishiteu", the strongest possible word for "love" often reserved for married couples. As such, his feelings are explicitly romantic love.
The cover of Chapter 45 shows Kanae vomiting and choking on rose petals. This is called "Hanahaki Byou", a sort of fictional disease/artistic symbolism where a person vomits flower petals as a result of Unrequited Love.
The Quinx have several characters suffering from unrequited love: Urie is implied to be in love with Mutsuki but Mutsuki is psychotically in love with Kaneki and desperately wants Kaneki to go back to the CCG with him. He begs him to go with him while pulling on his sleeve tightly. His behavior here makes one feel sorry for him especially since Sasaki/Kaneki is in love with Touka.
Matsuri is continuing on after losing his father because of his feelings for Urie who is just using him and is hinted to have feelings for Mutsuki. Matsuri dies by V with their love going unrequited.
In Assassination Classroom, Akari is hit with this trope both platonic and romantically. While undercover as a normal teenage girl, she came to truly care about her classmates (especially Nagisa) and their Badass Teacher. After a year of 'being' Kayano (and being defeated and depowered by her would-be target), the two personae start to clash, and it ain't pretty. Cue angsty screaming... well, she is a teenager.
Scum's Wish is essentially Love Hurts: The Manga, as every romantic entanglement in the Love Dodecahedron is unhealthy and/or painful, mostly due to almost everyone's feelings going unreciprocated. Hanabi and Mugi are pretending to date to numb the pain of their respective objects of affection (their teachers) not feeling the same way, while Sanae and Noriko are themselves devastated by those two, respectively, not returning their feelings. Then things go off the rails when Hanabi and Mugi develop feelings for each other, just in time for Hanabi to get her heart broken twice in a row when Narumi turns her down, and Mugi stands her up to enter into an ill-conceived relationship with Akane, who is mostly using him for sex. Meanwhile, Hanabi and Sanae start sleeping together, causing both of them intense guilt for a variety of complicated reasons.
In the light novels Guren recalls the love story 500 years ago that led the Hiragi Clan to scorn the Ichinose Clan. Apparently, two Hiragi brothers were in love with a beautiful Ichinose girl. They both competed for her love with the younger brother winning her affections. The older brother responded by raping and impregnating the girl and castrating his younger brother. He then banished them from the clan which led to them having to create the Order Of Imperial Moon for themselves. The scorned brother could have defeated them but chose to let them live so they, their children, and their children's children would forever be looked down upon and humiliated by others as the lowly Ichinose clan. Also, since the younger brother was castrated by the older brother he couldn't even impregnate his wife and she would end up giving birth to the older brother's child.
Mahiru and Guren fit the Star-Crossed Lovers scenario, in that since Mahiru is becoming a demon and is dangerous Guren has been ordered to kill her unless he can get her to stay with him. Given how far gone she is though...
The fact that Mika has been with vampires for FOUR years all alone, with the same vampire who killed his beloved family in front of him, AND with an undeniable thirst for blood shows just how sad his circumstances are...not to mention learning that his entire family were experimented on and that his remaining family Yuu (whom he is implied to have feelings for) is exposed to the organization currently leading the experiment. There's also the Star-Crossed Lovers implications with circumstances threatening to keep them apart. Also, if Yuu becomes a complete demon imagine how Mika who cares greatly for Yuu would feel. Vampires can only retain humanity so long as they have human connections after all. If, for arguments sake, Mika had his way right now and the two of them managed to run away, he would be able to save his precious Yuu-chan and maintain whats left of his humanity for the time being, but at the same time, Yuu wouldnt be able to retain his, knowing that he had betrayed and abandoned his comrades. Theyre pulling in the opposite directions, so first they need to find a compromise. Luckily, Yuu is charismatic enough to possess a potent power of persuasion, and Mika is sensible enough to be willing to believe in Yuu and compromise, so they quickly found the common ground in the beginning of the chapter. But if, say, Yuu becomes a full demon, Mika might be be even worse off than Crowley, because not only would he lose his precious person, he would probably have to kill demon!Yuu-chan with his own hands, and its obvious that it would destroy him emotionally.
Here's an example of a non-romantic love that hurts so much. In Fist of the North Star, young Thouzer was a loving boy that got no mean bone in his body and grew up adoring and loving his master Ougai like his own father. Cue the age where he had to succeed Ougai's branch of Nanto Seiken, but of course Thouzer would take it! Anything for his beloved father figure, right? Well, for the test to succeed the school, he has to fight a certain opponent blindfolded... Thouzer succeeded and to his shock, he learned that his opponent was Ougai, who set up the whole fight and knowing that he would not walk out alive because that's what the rules stated and he knew Thouzer was a worthy successor. Because of his immense love to his master, Thouzer was hurt so much that he started believing that love leads only to pain and weakness and he had no need of those and thus, a merciless tyrant that would enslave many children to death was born...
Tiger: Being gentle doesn't always mean love. When somebody hugs you it doesn't always means they love you. I will fight for Mister Faust, even if that means I have to die.
Lime: But why? Why die for somebody when you could live for that person?
Tiger: Because I have a heart...
Tamagotchi: Spacytchi had to learn that the hard way. Twice.
When he first met Agetchi, he was instantly smitten. In fact, she was all he could think about to the point where he ended up ignoring his own brothers! But at the very end, it turns out Agetchi already had a boyfriend, much to Spacytchi's dismay.
There was also Himespetchi, who ended up forgetting all about him even though they were together for a long time and fell in love with Mametchi.