I tend to over think everything to the point where I can't concentrate on anything. It effects my sleep too. I need to learn to let things go especially the past.
I wouldn't let the past go. Learn from it, yes, but don't forget it or think it's a bad thing. Without it, you wouldn't be who you are. As for thinking too much, I've found it helpful to keep a journal where I can write down whatever keeps me up at night. Something to try :)