I recently went through therapy for some touch up work. I was getting mad at someone because of their shenanigans. Basically the therapist told me "when someone pisses you off, they want you to get mad, so why would you give them the pleasure of getting mad?" After some discussion on methods for how to not get mad, I stopped getting mad.

Might be worth looking into whether it's worth wasting any brain cycles on whether or not to forgive someone.