Fan Works / Did Not Get The Girl

As this is an Ending Trope, unmarked spoilers abound. Beware.

In The RWBY Loops, Sun Wukong joins the looping crew only to find that Blake Belladonna, the girl he was crushing on, has been dating Yang Xiao Long for quite some time and the two are, in fact, recently engaged. Unusually for this trope, everyone is aware of the situation and somewhat sympathetic, attempting to help him move on.

Deconstructed in Girls Like Us. Barry repeats Oliver's comment about how "Guys like us don't get the girl" to Laurel. This is her Rage Breaking Point, and she spells out for him (and Oliver) that it isn't the vigilante lifestyle that will lead to them being alone, but the fact that they don't trust or respect "the girl". Oliver and Barry are both left shaken by it.

In The Masks We Wear, Zuko loses his affection for Mai as a result of his increasing similarity to his sister.

In New Hope University: Major In Murder, Saya Wild, the Ultimate Chess Grandmaster and the protagonist, appears to have feelings for Lucina Sorenson, the Ultimate Conductor. After the third trial, Saya rejects a kiss from Lucina, and shortly thereafter, finds out that Lucina has hooked up with Katy Thorson, the Ultimate Lesbian Romance Author. In the fifth trial, in which Lucina and Katy are the primary suspects, Saya deduces that Lucina is the culprit, resulting in Lucina's execution. In an alternative outcome to the fifth trial, Saya would have led the others to convict Katy instead. According to the author, Lucina would have become angry with the other survivors, only keeping in touch with Rocky after the killing games, but might have reconciled with the others later. Either way, Saya never had a chance of getting together with Lucina.

L in A Charmed Life. Though he has captured Misa at the end of the story and confined her to his bondage dungeon he would have much rather have had Light.

Played With in What If Goku Married Bulma?; as a direct result of the titular change, Vegeta never hooks up with Bulma or fathers Trunks. However, thanks to the multiverse, Future Trunks still exists, and still comes back to warn the heroes about the Androids, only to be very surprised to discover himself in a world where his parents never got together.

Spike in Beneath your feet, what Treasures; the story is about him dealing with his feelings towards Rarity via his treasure hoard, along with his knowledge that he will never get the girl.
A Diplomatic Visit: As seen in chapter 11 of the third story, Diplomacy Through Schooling. After all that ship tease between Moondancer and Twilight, and just as Twilight has decided to talk to Moondancer about taking a step forward in their relationship, Moondancer admits to Twilight that she's fallen for Trixie. The next story subverts it though when the trio talk things over, and decide to be a romantic trio.
In Featherfall (Equestria Girls) Rainbow and Gilda have a What Could Have Been argument that ends with Rainbow revealing her feelings for Gilda in the worst way possible: forcefully kissing her when she knows Gilda is with Sunset.

In Shatterheart Fai had been struggling with his eventual betrayal of the group and the exhausting effects of lying to everyone. By the time he comes to terms with his love for Kurogane, he can't do anything about it as Kurogane has already gotten together with Syaoran and he has to watch them became lovers.