I think letting things go is a challenge for most people, which means it's even more difficult for those of us with anxiety, OCD, etc.

Therapy has helped me a lot because it gave me a different perspective on the things I would obsess over. Medication can be great because constant worry just turns into a much more manageable type of worrying. (At least, that has been my experience.) Meditation can help too because it can be an exercise in watching your thoughts without engaging with them. I know, it sounds a little strange. But it can be great e.g., when I'm meditating, an old argument might pop up, but I just acknowledge the thought without replaying the argument and reliving it. Meditation takes a LOT of practice, but it can be good for the sort of thing you're describing.

There are still things still I can't quite let go of, but I don't obsess over them. (Not very often anyway.) It seems like the trick is to try out a variety of coping mechanisms and use the ones that resonate with you.

It's good that you're on here looking for advice and taking steps toward getting well. I hope you're able to continue that journey and ultimately find what you need so these thoughts don't interfere with your life quite so much.