Anger runs on the word should, and should is a word about debt. Write out what you believe this person owes you, then mark which of those they ever actually agreed to.

What the research says

In a randomised trial with 259 adults still troubled by something someone had done to them, a six week group programme aimed at forgiveness increased forgiveness and lowered both perceived stress and trait anger. Going at a specific grievance deliberately, over weeks, did more than waiting for it to fade. That was a structured group format, so the result belongs to the format rather than to one sitting with a pen.

Strength of evidence: one line of research. A single study, or a theoretical account.