I used to dwell on things for many years and regret even tiny instances where Ive hurt someone. One such case, a full 10 years later, I messaged a friend of mine and said, "Hey, I said something really hurtful to you 10 years ago and didn't realize it was as hurtful as it was at the time
I've regretted it all this time, just accept this apology regardless if you remember it or not" This friend took this apology really well, and since I did that, I don't regret trivial things as much anymore. At least not in an intense way. So for specifically forgiving yourself? Just remember that in most cases that the other party in whatever you regret likely forgives you so it's not necessary to spend time dwelling on it.