Accepting what you've done does not doom you to that forever. If that were the case than no one would ever be able to change or grow - they'd become stagnant.

Do monsters self-reflect on what they have done? Is that not the most human thing of all?

[Granted Carlisle means a literal monster which is different, and Genji doesn't have all the context here. But if he can forgive his brother for cutting off all his limbs, then Carlisle can forgive himself for being cursed.]