Two instances of remorse and forgiveness are Mozart’s reaction to his father’s death and his reaction to learning of Salieri’s help.
During the course of act 2, Salieri continues his campaign to destroy Mozart and promote his own music and career. His plotting seems to have the desired effect, as he manages to get Mozart’s Figaro banned, which makes him basically unemployable. Salieri decides to step up his assaults, aiming for total destruction of the man. The next time the two of them are together, Mozart is distraught, trying to figure out his next step.
Saying he wants to go to England, Amadeus tells Salieri that his father had refused his last request for money. He thinks his father is acting out of jealousy, he confides, and denounces him cruelly:
Underneath everything he’s jealous of me! He’ll never forgive me for being cleverer than he is. I’ll tell you a secret. Leopold Mozart is just a jealous dried-up old turd…. And I actually detest him.
As he giggles at his own naughtiness, the two Ventimelli enter, and together announce, “Leopold Mozart is dead!"
Mozart, initially concerned about his thwarted travel plan, is quickly overcome with remorse. There is an element of selfish concern, as he acknowledges that his father did understand the world and how to protect him from “the wickedness around,” and now no one else will. But he seems to genuinely regret his disloyalty, and talks past Salieri’s denials:
He watched for me all my life—and I betrayed him [...]. I talked against him [...]. I married when he begged me not to. I left him alone.
His father of course is not there to forgive him, and Salieri seems to offer solace, and a big hug, but Mozart does not accept, and cries out “Papa!”
In addressing the audience from his present vantage point, Salieri admits he showed Mozart no pity and continued to resent him for his superior musical talent, which revealed him as God’s chosen.
As his campaign wages on, he convinces Mozart he is helping him, and Amadeus begs for Salieri's forgiveness:
Oh forgive me! You’re a good man—I see that now—You’re a truly kind man. And I’m a monstrous fool [...]. You make me ashamed. You excellent man.
Salieri brushes aside his pleas but seems to accept them—only, the audience knows that he is insincere, so this is not actually an act of forgiveness but further infidelity to God.
This instance reverses the other, as Amadeus’s remorse over mistreating his father is genuine and well-founded, but he cannot make amends with the dead man; his remorse toward Salieri, although sincere, is misplaced, as Salieri has been working against him, but he can bestow it on the undeserving man who is still alive.