Rebecca's life after Josh broke up with her consisted of doing what her mother wanted, going to Mock Trial Camp, training to be a lawyer, going to Harvard and Yale, and just working on the time. When we see her ten years later, she is tired, depressed, and not sleeping. When it's revealed that she's getting a promotion, she gets a panic attack and tries to tell herself "This is what happy feels like."
"I Hope Josh Comes To My Party!" shows the moment in which a young Rebecca watches her father walk out on her and Naomi right in front of the only three kids who would show up to Rebecca's party. It's really hard to see the kid go from making a joke to being on the verge of tears and asking the other kids not to mention this to anyone at school.
In "I'm Going to the Beach with Josh and His Friends!", Rebecca is leaving a movie theater and notices that everyone else is with their friends, while she is alone. So she goes into a reprise of "I Have Friends" — but unlike the previous episode, "I Hope Josh Will Come to My Party" where it's played for laughs, this time, her complete lack of friends is played straight and it's shown just how sad she is because of it.
The whole of that episode is one punch in the gut after another: Rebecca on the Party Bus, unable to get anyone to talk to her except White Josh, who just wants to drone on and on about his exercise routine; then when Rebecca spots Darryl stuck in traffic and gets him on the bus, she's delighted to have someone to talk to at last, but Darryl immediately recognises White Josh from his gym, and immediately starts talking to him instead; then, the baffled response to her pole-dance, and Valencia laying into her, and Rebecca realising that she's ruining everyone's day; then she retreats to the bus bathroom and calls Paula to come and get her, but Paula won't do it until she admits she loves Josh, which Rebecca can't do.
The climax of "I'm Going to the Beach with Josh and His Friends!", in which Rebecca finally gets caught in one of her lies and has to reveal to the people on the Party Bus that she didn't move to West Covina because of a job offer, but rather because she had been depressed and wanted to be happy (she leaves out the part about Josh making her happy). It's obvious from some of the people's reactions that they don't exactly understand.
In "I'm Back at Camp with Josh!", Rebecca sings the beautiful "Dear Joshua Felix Chan"note Well, the tune is lovely, even if the language is as overblown as you'd expect from a hopelessly-in-love teenager, opening up to him for the first time...and he just starts laughing, assuming that she's having a good laugh at how ridiculous she was when she was 16.
Josh: Come on, read the rest of it. What else does it say? [laughs]
Rebecca: Oh, [rolling her eyes] it's like, the rest of it's like, more of it, like, the same stupid, like, "blahblahblah", y'know, "corn pops", and yadda yadda fudgsicles, and fireflies, it means nothing. Can we take the selfie? I think the sun's perfect.
In the selfie, he's grinning away, cheerful as ever, with a gorgeous sunset behind them; Rebecca stares into the camera with a look of hollow-eyed misery.
Afterwards, she goes to talk to the camp girls who have been cruel to her the entire trip without reason and when they start mocking her speech she just breaks down. For anyone who has ever been teased, her words are heartbreakingly close to home.
Rebecca: Why are you so mean to me? I've had the worst day and I feel so sad and small.
From "That Text Was Not Meant For Josh!", the song "You Stupid Bitch!", in which Rebecca spirals into self-loathing:
You ruined everything, you stupid bitch. You ruined everything, you stupid, stupid bitch. Youre just a lying little bitch who ruins things And wants the world to burn. Bitch. Youre a stupid bitch. And lose some weight.
And then she calls for the audience to sing along with her, saying "Yes, I deserve this!"
And it's mentioned Rebecca has this fantasy often.
Possibly the most painful part of the song is its realism—anyone who's suffered from anxiety or depression will likely recognize the way Rebecca's emotions spiral into complete self-loathing (including shots at unrelated things like her weight) over a relatively minor mistake.
In "Josh and I go to Los Angeles", Heather breaks up with Greg, telling him he should be with Rebecca. Greg realizes he does love Rebecca and goes to see her... only to see her making out with Josh. It doesn't help that Paula pretty much rubs it in his face.
In "I'm Going To Hawaii With Josh!", the scene where she's listening at the door while Josh and Valencia make up.
Basically all of the painful flashbacks in "Josh Has No Idea Where I Am!" including the one where it's revealed that her mother didn't rip her away from her dad, he didn't want her there and asked her mother to take her away.
Due to his insecurities, Greg completely and utterly blows it with Rebecca. She chose him and he still couldn't handle it. At Jayma's wedding, he basically tried to "play it cool" so much that he ended up getting drunk and finally, finally when Rebecca admits that she cares about Greg a lot, he replies, "That's cool." Afterwards, when he is throwing up on his couch, in tears, he says, "I love you. I should have said 'I love you, Rebecca.'" At that point, however, it's too late since Rebecca has already gone off to be with Josh.
The end of the first season, when Rebecca passes out and we see Paula sitting with her in the hospital, begging her to be okay. We knew Rebecca would be fine, but that scene reminds us how terrifying it must've been for Paula to see her best friend pass out for no reason and have to be rushed to the emergency room, with no clue what was wrong or if she'd be okay.
After Greg's disastrous wedding date with Rebecca, he sent her a text that said he had to visit his mom and that was the last she heard for a month. When Rebecca tries to find her, she finds that he didn't go to his mom, so where he is is a mystery. It's finally revealed that the next morning after the wedding, Greg was still drunk and was pulled over by a cop, who sentenced him to go to AA meetings where he realized that he was an alcoholic. All of last season's scenes of him drinking or getting drunk are now shown in a completely different context, now that he admits that he is addicted to alcohol.
In Season Two's "When Will Josh See How Cool I Am?", Rebecca meets Greg for the first time since he passed out at the wedding and he admits that the reason he was arrested for DUI and has faced his own alcoholism to the point of regularly attending AA meetings is that he was driving to her house to tell her that he loved her.
In "Why is Josh's Ex-Girlfriend Eating Carbs?", Valencia breaking down and admitting to Rebecca that her and Josh's relationship had died long before she actually broke up with him.
In "Who's the Cool Girl Josh is Dating," Paula misses Scott's first barbershop quartet performance in front of his co-workers (which was very important to him). Paula tries to apologize, but a very hurt Scott simply replies with "I dont care whose fault it is. I just feel bad right now."
The End of Season Two we learn why Rebecca went to Harvard and Yale. She had an affair with a married professor, he broke up with her, she attempted arson, and ended up with a restraining order preventing her from going to Harvard Law, and spent time in an institution. We learn this just after Josh breaks up with her, and after father finally shows up only to ask for money. Paula has to talk Becca down from a cliff.
The fact that Santino Fontana has left the show and his song "It Was a Shit Show".
His getting on an escalator and finally disappearing into the light at the top, while Rebecca tearfully watches him go.
"Rebecca's Reprise". Doubles as heartwarming.
The fact that the events of the season two finale nearly drive Rebecca to jump off a cliff. Paula has to talk her down, and is near tears herself, obviously terrified she's about to watch her best friend commit suicide. Valencia and Heather are both also present, unable to do anything but watch with fear, unsure if Paula can save Rebecca. This is notably one of the only times we see genuine emotion from Heather, and Valencia looks like she's about to cry.
The reaction of Rebecca's parents to the scene makes it worse While to her credit Naomi does at least look panicked, Rebecca's father doesn't even care and calls her crazy. Paula, as messed up as she is, is actually the closest thing Rebecca has to a good, caring parental figure.
Rebecca buys her dad fourteen Father's Day cards, for all of the ones she missed, and gives him money for his other kid's braces. It's pretty clear to the audience that he's just using her, but she's so desperate for his love, she's completely oblivious. The lines about him in "Rebecca's Reprise" are pretty heartbreaking too, even if she thinks they're uplifting.
Daddy's little girl, princess of his world, That was never something I knew before. But now that I'm a bride, he'll look at me with pride. Because my daddy will love me, And then in a wonderful way, Everything in the past will just fall away.
The start of season three has Rebecca depressed, alone, and holed up in her hotel room for weeks on end. Even if she crosses several lines later on, it's still sad to see her so heartbroken.
Rebecca writes a completely false Hit piece on Josh, and his friends turn against him. Worse is that Josh found out he's been a victim of stalking and harassment and his friends refuse to believe him.
Rebecca lashing out at the entire gang when they find out the full story of her past and try to have an intervention. After spending the last couple years with these people, she knows how to hit every single one where it hurts the most and leaves them all in tears.
The "revenge on Josh Chan" arch concludes with Josh telling Rebecca if she ever comes near him again he'll call the police. With Josh completely and permanently out of her life, she's completely lost, winds up sleeping with Greg's dad in a very messed up attempt to feel like she used to, and in the end breaks down and moves back to New York. The "End of a Movie" song while having some comedic moments is overwhelmingly a sad ballad on how Rebecca keeps repeating her mistakes, how messed up the characters are, and how empty and broken Rebecca really is.
Rebecca overdosing on the plane. And before that, when Naomi discovered she was looking at suicide websites.
Valencia breaking down in tears when she finally admits to herself and the others how terrified for Rebecca she has been in the wake of her suicide attempt.
Darryl breaking up with White Josh.
Darryl's utterly heartbroken reaction to finding out he wasn't having a baby.
The song "A Diagnosis" is very emotional, especially if you've dealt with mental illness personally.
Rebecca admitting to Dr. Akopian in "Nathaniel and I Are Just Friends!" that she's scared that if she doesn't have someone to love her, she'll cause damage to herself or others.
Paula's reaction in "Nathaniel is Irrelevant" when she found out that Rebecca lied to her about Trent blackmailing Paula.
"I'm So Happy For You":
The whole episode has this layer of sadness to it as Rebecca and Paula face the fact that they'll be separated from their loved ones.
When Tommy says he'll join a Peace Corps-like organization, Paula tries to put open a happy face to say she is proud of her son, but she's just barely holding back her immeasurable sadness, and leaves the room when it's clear she'll break down crying.
Nathaniel confessing to Heather that the only people who he's ever really loved are his late nanny Heidi and Rebecca, and he's lost both of them.
"I'm Almost Over You":
In the episode Nathaniel daydreams that he's in a romantic comedy and that Maya is the quirky love interest to his character. At the end of the episode, like in most romantic comedies, he rushes over to Maya to declare how he feels, but Maya starts talking about how they had "fun" and kissed in the elevator...and suddenly she isn't Maya, but Rebecca, because he was always thinking about Rebecca. Daydream Rebecca then points out that she's finally happy now and Nathaniel has to let her go. Nathaniel looks crushed as he says, "Really?"
Rebecca getting angry at Greg to the point of having a falling out with him in "I Need A Break" because he isn't enjoying himself at the waterslide park. The thing is, Greg is clearly being his absolutely best self, trying to enable Rebecca to have a good time even while they're doing something he hates, but he's still himself - he isn't a happy or playful sort of person, and he never will be, and Rebecca is essentially telling him that that means that his best efforts aren't enough.