Tell yourself "i'll think about it later" and force yourself to go outside and do something different. Go to the library or book store and get a book on "bullying back" your thoughts. Rent a movie, play a video game, call a friend, help someone in need, shop, read a gossip magazine, play loud music over your walkman-radio, create something artistic, re-organize your closets -- anything to break the cycle. Every time the thought comes back, keep saying "Oh you again. I said I'll think about you later..." and keep doing the other thing. It's hard I know, because when something bothers me, my thoughts go back to it over and over again. Sometimes I'm surprised at how creative my brain can be to sneak the annoying thought back into an unrelated topic. I usually use humor. If it's a "person" that keeps popping into my head, I imagine that they are wearing a clown suit or slip up (I'm partial to slap-stick humor). When I was worried about my daughter's health (even after she was BETTER!) and could think of nothing else, I wrote it all down in a notebook and said "there, that's it. Case closed." I wrapped it all nice and tidy and put it in the file cabinet. You could arrange to talk to a therapist and unload your thoughts to him/her. Try these things consistantly, and know it will get better.

Good luck -- and remember that this will pass.