I'm a worrier and an overthinker. And it used to cause me problems but I have changed my behaviour lately.
First is the idea what you choose what you think about - always take responsibility for everything in your life - once you have accepted that you can change! Otherwise you're in denial
So when I have a problem I am thinking over I have to really be aware of whats going on in my head. When I start going over the same ideas over and over again I know its not good! At that stage I go over my solutions or possibilities and I get realistic - ones that are not going to happen I stop considering. Just forget them. Then if I am left with a few I'll write them down and see what is most feasible - maybe sleep on it. But I make sure I have formed ideas not just abstract things in my head.
Worry - well that catches me sometimes still, but I have a very simple thing to deal with this. You look at your problem - like lets say your horse isn't rugged and you're worried about it. You ask yourself, can I do any action to help the situation? If yes, then do it. Right away. Go rug your horse. If you can't do anything at all - then stop worrying its not going to make a difference.
Like worrying after a test - is there anything you can do? No, so forget it, what is done is done. Wait for the future and when it comes you can make decisions to change outcomes, but for now that time has passed.
It may sound a bit tricky to just let it go, but its not really. I think its not these issues that bother us - its the uncertainty. Could I have done better? What can I do to be better? If I had answered this way instead what would it be like? What are these people thinking? Will I ever be better than my friend at this? etc. Although look at your worries, give them due consideration and then decided. Either decide that you can do something, and then do it, or decide you can do nothing, and move on. Everytime it comes to mind, push it aside, you've already made your decision.
That's just my advice.
I developed these ways of dealing with doubt, concern and guilt as an overall tool to combating depression, which I experienced from my mid-late teens. It's always something that lurks over my head, and if I don't take responsibility for how I feel and think and react then it can grab hold really easily. Take my advice now - don't be a slave to your emotions, experience them but don't let them control you. You are not how you feel. You and you alone are responsible for anything and everything you feel, and while this may be a daunting concept, its also extremely powerful.