Continue reading the book "The Art of Thinking Clearly", by Rolf Dobelli, on overthinking.
Thinking is good. But overthinking is not. It can prevent you from carrying out your plans, or take away your courage of doing it.
How to tell if you are overthinking?
See if you've spent too long on thinking about a problem but made little progress. See if you sense more and more worries instead of ambition to do something.
How to avoid overthinking?
Try to use some mechanism to break it, such as using a timer. If you are dealing with a very important problem but having trouble to get around your head. Give yourself a specific time period, and force you to go out and do something (ask around, try different methods) if that time is over.
When to think with your head and when to go from your gut? For things you've done many times or simple enough, you probably have more courage and good to go from your gut; for complex tasks such as making an investment, put more thinking into it, but do not go overboard.
Sometimes, giving a solution try is much better than thinking about the problems too much.