In All My Sons, protagonist Joe Keller knowingly sold defective aircraft parts to the Army during WWII, leading to the deaths of several pilots, but escaped punishment by pinning it on his partner. He'd succeeded in rationalizing it away by persuading himself that he did it for his family, but those rationalizations start to collapse in Act II when his younger son (who'd been a captain in the war) finds out and reacts in horror. He still manages to vociferously defend himself in Act III, however, until his son's fiancée produces a letter showing that his older son, presumed missing in the war, actually killed himself out of shame when Keller was initially accused. He says he'll go with his son to the police station, goes inside (offstage) to get his coat, and we hear a gun fire.
The Wizard of Oz has one at the end of Act 2 in Wicked when Glinda confronts him with the fact that the Wicked Witch he has successfully ordered murdered was actually his daughter.
Elphaba has one in "No Good Deed", when she watches her beloved (Fiyero) get dragged away to be tortured and interrogated for talking to her as public enemy number one. She tries to cast a spell to save him, that inevitably turns him into the Scarecrow, but when it seemingly fails, she crosses the Despair Event Horizon and declares that "no good deed will I do again".
Antigone, when Creon has condemned his niece to death and driven her to suicide, which causes his son to commit suicide, which causes his wife to commit suicide. This is made worse by the fact that he had already gone back on his decision to kill her before he knew of her death.
In The Trachiniae, after Deianira sees the effect of her "love potion" (a centaur's hydra-poisoned blood, actually) on the scrap of cloth she used to smear Herakles' garment with, she starts to realize what is really going on and begins freaking out. When her son Hyllus arrives to blame her for killing dear old dad in the most painful way possible, she really loses it.
Handel's English opera Hercules, based on The Trachiniae, turns Dejanira's realisation of her 'fatal error of misguided love' into a full-blown operatic mad-scene, complete with hallucinations and wild shifts of mood. All meticulously represented in music. It's terrifying.
Jean Valjean's soliloquy after being saved by the Bishop in the musical adaptation of Les Misérables begins with almost exactly this line. "What have I done, sweet Jesus what have I done? Become a thief in the night, become a dog on the run? Have I fallen so far, and is the hour so late, that nothing remains but the cry of my hate?" Valjean goes on to explain that after a lifetime of being convinced that he was worthless and the world pitiless, he has met a man who is truly good and selfless, robbed him, and not only escapes punishment but receives further kindness when the Bishop gives him a pair of silver candlesticks Valjean hadn't originally stolen, along with the stolen goods.
Another example comes from Javert's life being saved by Valjean. The cognitive dissonance of being saved by a parole breaker, someone he previously thought to be no more than a cold-blooded criminal, is too much for him to live with. He was forced to see Valjean and those like him as people for the first time, which called into question the ideal of justice which was more or less the pillar of his existence. Both soliloquies are actually set to the same music.
Valjean gets another later, albeit one less earth-shattering, when he realizes he allowed Fantine to be fired, which forced her into prostitution to pay for her child's upkeep. "Is it true what I've done? To an innocent soul?" This time it prompts him to pay for Fantine's medical treatment and, when she dies, take in her daughter as his own.
Eponine gets a minor one following "The Robbery", when she sees Cosette after so many years and is briefly forced to face the years of abuse that she (Cosette) suffered at the hands of Eponine's parents while Eponine stood by.
Mrs Erlynne from Lady Windermere's Fan has a moment when she realizes that her blackmailing of Lord Windermere has made his wife think he's cheating, prompting her to leave him. Worse, Mrs Erlynne is Lady Windermere's mother, who made exactly the same mistake 20 years ago, and is now watching her daughter repeat her own history.
Like all versions of Carrie, in The Musical, Sue Snell feels guilty for humiliating Carrie along with the other girls, and apologizes to her. However, in this version, Carrie rejects the apology, feeling it's not genuine (and in a way, it isn't, as Sue's just trying to relieve her conscience). This forces Sue to realize how horrible she's been to Carrie all these years, and how much pain Carrie's been through as a result, prompting this reaction from Sue, who then becomes The Atoner. Best expressed in the song "Once You See."
She... she's always been there. I... I never knew.
I felt as though this girl revealed herself to me.
In Pokémon Live!, Delia Ketchum has this reaction after she tells Ash about her past; the revelation combined with Team Rocket stealing his Pikachu made Ash surrender to them to be taken straight to Giovanni—something Delia desperately tried to avoid.
In Albert Herring, Nancy laments in three stanzas having helped make Albert Unsuspectingly Soused when the day after he's missing and feared dead by some. Her lover and accomplice, Sid, is unrepentant.
Little Shop of Horrors eventually leads Seymour to think this. After Mushink's and Audrey's deaths, Seymour realizes what Audrey II is and what it was doing on Earth from the beginning.
The thrill killer Richard in Thrill Me finally grows a conscience in "Afraid," when he realizes he has no excuse or justification for what he's done. (That is, murder a twelve year old for the hell of it.)
In the opera Aïda, Radames is so dishonored that he loses the will to live after he unwittingly betrays the Egyptian battle plans to the king of the Ethiopians.
Amneris has it too, as all she did resulted in her beloved (along with her rival) being buried alive. And she's just behind the door during the trial of Radames, desperately crying he's innocent but unable to fix anything at this point.
At the end of Verdi's Il Trovatore, di Luna has a very dramatic one of these when Azucena reveals that he has just had his own lost brother executed. Not to mention he's also driven the woman he loves to suicide.
La Traviata's Alfredo pitifully delivers a version of this line after he publicly humiliates Violetta by chucking money at her as payment for her 'services'. It's only worse to the audience who, unlike Alfredo, know that Violetta left him for noble reasons and still loves him desperately.
In Rimsky-Korsakov's The Tsar's Bride, Gryaznoy gets a heartbreaking one when he sees what his attempts to get Marfa have come to. Marfa is dying from the poison he slipped into her drink himself, believing it to be a love potion, for it was substituted by his former mistress whom he had treated dreadfully. Worse, after he successfully framed and killed Marfa's beloved Ivanand announced it to Marfa, she has gone completely deranged and mistakes him for Ivan in her delusions. Gryaznoy cries in anguish that it's him who must pay for every tear of Marfa's.
In Hamilton, Aaron Burr is horrified after fatally wounding his former friend, the protagonist Alexander Hamilton, in a duel. Especially because Hamilton aimed at the sky, intending to spare Burr. He cries "Wait!" and tries to go to his dying friend but isn't allowed near the body.
I should have know the world was wide enough For both Hamilton and me
Although Hamilton initially believes he is correct in releasing the Reynolds Pamphlet, he quickly realizes his mistake when Jefferson, Madison, and even his sister-in-law Angelica abandon him.
A strong theme in "It's Quiet Uptown": not only was it Hamilton's now-public affair that drove Philip to duel George Eacker, but Hamilton even encouraged it when he came to him for advice and gave him his set of dueling pistols. Both lead to Philip's death.
Dream Girl has a Dream Sequence with Georgina as a Fallen Woman who is Driven to Suicide after Clark tries to have her reported to the police. When Clark finds out that she is dead, he is overcome with remorse and tells the officer to arrest him as a murderer: "I killed the girl as surely as though I stabbed her through the heart."
Janaček's Jenufa practically embodies the trope. First there's the end of the first act, when Laca cuts Jenufa's cheek in a fit of jealous rage and breaks down in remorse a split second later.
Laca: What have I done? Jenufka! I have loved you since childhood! (Repeats it until the curtain falls).
Then Jenufa's stepmother drowns the girl's illegitimate baby to save her reputation. The act isn't shown but when she reappears onstage she is already half-crazed with guilt, and it gets worse.
Additionally, both of them go through it again in the finale when the baby's remains are discovered. Since it was Jenufa's scarred face that made her lover abandon her and the child, Laca realizes it was primarily his actions that led to the disaster.
Saint Joan: John de Stogumber, an English cleric, spends most of the play determined to get Joan burned as a heretic, and speaks forcefully against her at her trial. After witnessing the burning and gaining an understanding of what a cruel death it is, he's so remorseful he has a Freak Out!. In the Epilogue, he's retired to become The Vicar of a small village whose inhabitants regard his sermons on the importance of understanding the consequences of your actions as a charming foible.