I have that same problem. In fact, I asked a question about this a while back. What's been helping me is keeping myself occupied.. the busier I am, the less I think about things. It's so hard, I find myself losing sleep from thinking too much, but that's just how I am.. I break everything down and analyze until I drive myself crazy enough lol
Focus on only the significant things that effect you directly. Do not worry about everyone elses problems, issues and opinions. Those people are not going to pay for your trips to the salon to dye away the grey hair you get from obsessing over everything. Worry about your school/career decisions, family and most importantly your health.
when you realize that your doing either of one of these two things, stop yourself, and realize that your doing it.
however if this is anxiety issues that youre dealing with, you might want to see a doctor cause they can prescribe you some medication or something....
you can't, its your nature and its who you are if that is the kind of person you are, nothin to be ashamed about