Unbalanced by Rival's Kid

One generation back there was a Love Triangle. Now the rival who lost the competition meets —and has to interact with—the child of the other two. This causes emotional chaos. The character loses the ability to act calmly (or even adequately) whenever a situation arises related to the kid. The effect is greater when the loser is an otherwise calm, Magnificent Bastard or flawless hero proven Not So Stoic and Not So Above It All.

If the progeny is of the opposite sex and resembles the contested person too much—often the case—it's even worse. Expect a Wrong Name Outburst. Not only is it a reminder, one sees the image of the lost love, their would-be kid, an image of the rival, or a second chance, and so on—often two or more of these at once. Usually female kid and male ex-rival.

Anime and Manga

 * In Hayate the Combat Butler Mikoto Tachibana probably had this reaction upon meeting Nagi, who greatly resembles her mother, who Mikoto had an unrequited crush on. Though both Nagi's mother and Nagi herself seem to be not interested in the affection shown because they aren't interested in women, and the arranged marriage was said to be set up by the grandfathers, Mikoto probably had no issue with her son being set up with the daughter.
 * Since it's also been implied that Hayate resembles Nagi's mother (and her father), this could also take place if Mikoto were to meet him at some date in the future.
 * In Zoids: New Century Zero, Dr. Layon and Dr. Toros loved the same woman, who eventually became Dr. Toros' wife, and mother of his son and daughter. Layon now hates Toros and never meets his son, but actually likes the daughter, who reminds him of the presumably dead Mrs. Toros. Not in that way.
 * Fate/Zero has Kariya Matou, in a sense. He is the Unlucky Childhood Friend to Aoi Tohsaka that keeps hanging around even though she has two daughters with Tokiomi. Even so, he also cares about Rin and Sakura. In fact, when he discovers what Zouken is doing to Sakura, he agrees to enter the Holy Grail war on the behalf of the family he'd severed ties with ten years earlier just so he can earn Sakura's freedom. Unfortunately, he fails and ultimately.

Comic Books

 * Elf Quest When Rayek meets his former lovemate's children.

Literature

 * Songs and Swords Forgotten Realms novels: Elaith Craulnober and Arilyn "Moonblade" < Amnestria Moonflower + Bran Skorlskun. "Amnestria!" was Elaith's first word when he saw Arilyn. Unusual in that later.
 * Keturah's daughter and in Counselors and Kings. Mostly in a benign "would-be child" way, but this still motivated more activity than is normal for him and even an uncharacteristic murderous rage when he discovered that she's in danger.
 * Severus Snape in Harry Potter treated Harry the way he did because of this. His dad had a mutually antagonistic relationship with Snape. Further, in Harry Potter we learn
 * Snape was a Sadist Teacher anyway (just ask Neville), with a favoritism toward Slytherin students (especially Draco), but there is no doubt that he treated Harry even worse than the norm.
 * The Count of Monte Cristo: Edmond determines to use Mondego's son as a tool of revenge. How this turns out varies among adaptations.
 * Briefly, in The Great Gatsby, where Daisy's child serves as a symbol to Gatsby of the reality of her marriage to Tom.
 * Warrior Cats: Ashfur was in love with Squirrelflight, who ended up dumping him in favor of Brambleclaw. Ashfur had some problems accepting this. He even arranged to have Squirrelflight's father killed so she'd feel the same pain he did. The attempt failed though, and Ashfur was never suspected for the attempted murder for a while. He eventually received an apprentice, though, who managed to flare up his issues all over again by being Squirrelflight and Brambleclaw's son, a living reminder of the mate he couldn't have and the future he was denied. They don't get along. While they are never outwardly rude to each other, Ashfur mostly gives Lionpaw the cold shoulder and doesn't try to bond with his apprentice the way all the other mentors do. When Lionpaw demands more intense training, Ashfur takes this request very specifically...by unsheathing his claws and fighting Lionpaw as if it were a real battle. Ashfur is eager to finish Lionpaw's apprentice training quickly so that they won't have to see each other eye-to-eye anymore.
 * From A Song of Ice and Fire: in the first book, preteen Sansa Stark meets Petyr Baelish, the Unlucky Childhood Friend of Sansa's mother Catelyn...whom Sansa greatly resembles. He creeps her out in their initial meeting by pointing this out and touching her hair, and things have only gotten squickier now that she's posing as his daughter.

Video Games

 * A chapter in the tutorial stages of Fire Emblem has Lyn and company seeking the aid of the Marquess of Araphen, a suitor of Lyn's mother before she eloped with a Sacaean nomad, which sparked in him a hatred of Saceans in general. At first the Marquess is willing to help, but after he meets Lyn in person, he withdraws his offer, saying that she is "tained by the blood of Sacae".

Web Comics

 * Gunnerkrigg Court has Sir James Eglamore and Antimony Carver—the daughter of his old flame Surma and the rival and friend Anthony Carver. He falls out of his armor time and again. Word of Tom says that the strong resemblance is one of the factors—in case the Wrong Name Outburst wasn't telltale enough. It seems that he got better with the help (sometimes unfriendly) of Annie and his own colleagues. Renard—who "fell desperately in love with Surma" as well—was shaken too, but in a different way; eventually he settled on being an unofficial Parental Substitute.
 * On the other side of that unlikely pair was Brinnie. Who departed before Surma's change of heart and upon return was stunned by such news. At least, it took her only three panels or so to get over this and resume reunion like nothing happened.
 * Girl Genius got Baron Klaus Wulfenbach and Agatha Heterodyne < Lucrezia Mongfish + Bill Heterodyne. Circumstances like how Klaus and Lucrezia parted or that he knows —and just how dangerous anyone of this family can be—do not improve the situation.
 * And that's not even getting into how

Western Animation

 * Vlad Masters, to the eponymous Danny Phantom.
 * Actually sort of a subversion / inversion—Vlad actually likes Danny (in a twisted, Affably Evil way). His big dream is basically to steal Maddie back from Jack and make Danny (and Jazz, to a lesser extent) his own adopted children, something which they, as you can imagine, find rather unsettling.
 * An episode of The Proud Family had this where the failed rival in question was one of the protagonist's teachers (the resulting couple being her grandparents).
 * Avatar: The Last Airbender Plays With this trope: Pakku becomes quite emotional when he realizes that Katara's grandmother was his Runaway Bride, but ultimately this makes him warm up to Katara and agree to teach her Waterbending. Also, . Also not a normal example, since Pakku presumably never knew the "rival" (Katara and Sokka's grandfather).
 * Played With in The Legend of Korra: Lin Beifong has yet to interact with the children of her ex-boyfriend, Tenzin, but takes out her resentment on Korra, his live-in student, surrogate family member and the Reincarnation of his father Aang, with whom, by Tenzin's account, she got along quite well.