Advice on how to become less angry?

I cut off a friend early this year after he did many many many uncomfortable and weird things. He betrayed my trust in a lot of ways and I find myself thinking of him and getting angry pretty often. I communicated with him before cutting him off that what he did upset me but I don’t think he cared. He’d always do something new that would upset me or even do something he KNEW would upset me. I get so tempted to send him a long angry text explaining how the things he did were not okay but I know there’s no point. I’m not gonna feel better if he apologizes and I don’t wanna open that door again.

I know eventually I’ll stop thinking about him so often but I feel like I’m always gonna have this anger. I think the experience taught me that I don’t have to be nice to people who repeatedly cross my boundaries. I was way too nice to him and I ignored so many red flags.

He just still really upsets me and I don’t like getting this mad. The anger goes away once I get distracted by something but then some other thing will remind me of him and I’ll get pissed off again. I guess I’m just wondering if anyone has advice for dealing with this other than like “do your hobbies” or “chat with a friend”. I know those things help but the anger always comes back.