I don’t want him back but how do I deal with my anger?

In time you’ll get desensitized to his presence and thus indifferent, but until then there are a few ways

It’s easier to tolerate people you don’t like if you resist thinking they’re “all bad.” No matter who I can’t stand or dislike I can always think of some of their good traits. Or as a last resort genuinely feel sorry for them. No, this doesn’t result in wanting to associate with them, nor forgiving them if they screwed up etc. I still want nothing to do with them but it does warm me up towards them instead of stewing in anger. And that balance makes me tolerant enough to be content in giving them basic human respect, wishing them well but all with a “reserved politeness.”

Stop hoping you won’t run into each other or he won’t appear in your eyeline. A lot of the times how you feel about something is based on expectation (and a bit of letting go of control). He’s going to be there and you need to adapt yourself to that new reality. You can even make it as a challenge for self growth. Good luck