The resentments I have trouble letting go of are the ones where I had no part, the ones that hurt me when I wasn't even looking. Those are the ones that I take my lesson from and try to be grateful for what I learned, then let the rest of it go.
They can be sneaky though. Sometimes I'm not even aware of resenting something until I get that niggling twitch every time a person or situation is brought up and then I know that there is something there that I need to tend to, usually a well-disguised resentment. These ones catch me by surprise.
Hugs
Ann