Honestly, while some may deride the protagonist. He's honestly realistic. How can you forgive someone who tormented you that long? How can you, after spending so long building this idea of revenge in your head, just let that go? You want the bully to stay the same, so you can make them have their comeuppance. That's the story.
But, he didn't get anything. He didn't get that satisfaction.
It's not fun, nor satisfying to get his revenge on her, because that's not his bully anymore. And she accepting his hatred is just another form of pity to him. He didn't get a "I got revenge on my bullies, and now my life is cool" moment, someone else did it for him, and he didn't even get to see the pleasure of seeing his bully get bullied in return. Another group of bullies got involved, left his life for years, and now his bully is remorseful, due no part of his.
He feels cheated out of his revenge, but can't bring himself to do anything about it, because neither he, nor his bully, are children anymore. The one he wants revenge on is that bully from his childhood, but that girl only exists in his memories.