Two things. First, not all smart people are overthinkers and not all overthinkers are smart.

Second, I find that a great way to change one's self-damaging behavior is, rather than the therapy that is often recommended, to try to be as much as possible, relatively speaking, in the company of people who behave the way we would like to.

For the person who wants to exercise, but for some psychological hang-ups, can't, the company of people who exercise tends to be much more effective than finding out the root causes of the behavior. The same for thinking too much, eating too much, not being able to talk to other people.