the half chapter makes it harder to parse, but from what I gathered -
He's finally going to
properly confess/ask her out since they've been operating under the fake/coerced confession.
But he's also remembering how he was fucked over by someone at his previous school before transferring back, where (as best I can figure) he was called to a classroom after school by a girl, ostensibly for a confession, only she instead grinned, starting screaming that she was being assaulted, and no one believed that he
didn't do that, to the point his mother had to shamefully apologize/possibly pay restitution for what he'd been accused of, and I'm assuming he was ostracized/bullied/harassed the rest of his time there before he came back to his hometown to this school, where he wanted nothing to do with anyone until all of this went down.
And the FMC saying "I thought you might be like me, also deeply hurting", she is stating that she's clocked that he's got heavy shit under the surface, but that she doesn't mind, and likes that he didn't treat her like everyone else - and that she wants to be with him.
And he's got a bit of dissonance going on, because the
last time this happened to him, his life was ruined and he had to switch towns/schools, but he's wanting to take a chance and believe in her and them together, here and now.
No idea what the deal was with the girl from the past, but.