Dear Lou Lou,
You have a very common problem. If I tell you to NOT think about a giant pink elephant, the first thing that will happen is that you will think about it. However, studies have shown that if tell yourself to just "stop it" whenever something you don't want to think about pops into your mind, it will eventually get better. Don't tell yourself not to think about it, but just say "stop" whenever it happens. You need to say it very strongly. It can be said silently. Your problem will improve over time.