It may seem that I am a sort of sneaky advertiser. I promise you that I am not.
There is a book called "Thoughts and feelings", written by Matthew McKay and others. It deals with many psychological problems. One of them is obsessional thinking. And that is what you are suffering from.
Psychology has researched a lot of methods on how to deal with this issue. The book ''Thoughts and feelings" summarizes the most effective techniques known at this date to deal with this problem (and many other problems). It also explains how to use them to help yourself without the involvement of a therapist. The techniques to deal with obsessional thinking are:
- Thought stopping (developed by Bain (1928) and Joseph Wolpe late 1950s)
- Flooding (developed by Thomas Stampfl, Zev Wanderer, Paul Salkovskis and Joan Kirk)
- Worry control (developed by O'Leary et al 1992, and Brown et al. 1992)
You probably could find some information about these topics on Google. I do, however, recommend buying the aforementioned book. It's a small investment (less than 20$ I think) that will gain you a huge increase of knowledge about scientific research on how to deal with disturbing thoughts, emotions and habits.
Hope this helps,
Francis