Consider your longer-standing resentments. Isn't part of the anger that the person who hurt you likely does not even remember the offense, or even, may not remember you? They hurt you and move on, and if we were to confront them with the data they might have no recollection. Or, they might consider it unimportant and you quite petty for holding onto it for so long. Perhaps worst of all, they might remember the event but with an entirely different spin, one that generally absolves them and accuses you.
First, remember that there are a hundred people out there who feel the same way about you. An offhand comment, a slight you have long forgotten, still animates them. And they will likely never tell you now. So for that reason alone, it is best to lose the resentment - you really don't want anything like a fully-remembering, fully-just world to descend upon you.
But there is a further reason to let loose your grip on these things. You remember them inaccurately. While it may seem vivid to you, the hurtful words seared into memory, they are in fact not seared. The more often you remember an event, the more you change it in light of later information. There is ample reason why this is psychologically good for you, allowing you to understand your world as a whole rather than a series of unrelated events, but it does deceive. It is not foolproof, and you may have for years trended in the direction of changing the events in memory .01% each time we remember. If we could magically retrieve a video someone took of the scene, we would find that the quote seared into memory is in fact approximate...that the conversation did not go in the order you remember...that there were other things said which you have forgotten, which might change the blame/credit balance.
How do we know this? Two things strongly point to it. When people describe remote, even innocuous events, not knowing there actually photos (or even a recording) of it remaining, they find their accuracy is not good. If we are not too strict, we usually find that people got the events approximately correct, not as if they made up whole sections of their history, but there are surprises. Try it yourself with a group watching old video sometime:
A: Why the heck was Aunt Daisy even at that party? She's from the other side of the family.
B: She was up visiting her sister, wasn't she?
C: Yes, her sister was in the hospital and we thought it would be a nice distraction.
D: I'd forgotten how short she was.
A: Well, you were only 12 when she died. Everyone looks tall when you're a child.
Secondly, we know that emotionally-charged events are especially poorly remembered, even a few days later. In particular, people leave things out and get them in the wrong order.
I write this as a person who is conscious of carrying very few resentments when I am in a good mood, but constantly find that stray comments or new versions of the old injury trigger anger. I want to replay the event and do better this time. Not better in the sense of being kinder and more adult, but better in the sense of being more effectively cutting and victorious.
It pays to remind myself that what I remember is almost assuredly deceiving to me, even if I have some of the events right.