I understand the re-living past mistakes thing, too.
It's not as easy as relaxing and clearing the head - there is something more fundamental going on than just a racing mind. As Kate54 says, it seems to be part of the ageing process. Look at the number of books and films which use the structure of an old person looking back over their lives.
I know it solves nothing, but that doesn't mean I can make it stop. I think the only people who don't worry about hurting others are psychopaths, aren't they?
Annoyingly, if I knew that there was someone lying awake worrying over things they had said or done to hurt me, I would (with only one or two exceptions) forgive them and tell them to forget it, as I definitely don't remember. The chances are that the people I am thinking about would say the same to me.