Most grudges have an expiration date. Stop keeping them in the fridge.

A resentment kept in working order takes maintenance. It has to be revisited, retold, and kept cold, and without that effort most of them lose their charge on their own within a reasonable stretch of time. Noticing that you are the one doing the upkeep changes how the whole thing looks.

Pick one you have been maintaining and simply stop rehearsing it for a month. You are not required to forgive anything or to contact anyone. Leave it alone and see how much of it is still there when you check.