Grudge Match

I used to be the world's best grudge-holder.
I didn't MEAN to be, but according to my parents (who never fail to remind me that I "only remember the bad stuff")(which is weird, because clearly they're the ones remembering bad stuff that I've LOOONG forgotten), I could hold a grudge forever.

But the older I get, the quicker I am to forgive, and it always surprises me when people don't accept an apology.

And although I forgive, I can't always forget. Which is ALSO weird because I am growing more forgetful all the time.
It's not like I sit and BROOD about it or anything, it's more that it just hangs around in my head and gives my belly a squinch of anxiety every so often.

I try and try to let things go, but I find I am harboring some resentments.

How do you let stuff go, when you think you've sincerely forgiven someone but it WON'T GO AWAY?
Are you surprised when you apologize and people keep beating you up over the issue?