I get this - a lot. For me personally, it's related to my OCD. Something horrible will pop into my head and I then have to perform a ritual to sort of cancel it out. Sometimes I become so preoccupied with NOT thinking about things that I think about them more and the cycle continues.

The best thing that combats it for me is relaxation and breathing exercices. I try to imagine a blank white wall and everytime a picture or thought starts to develop i get a brush and paint over it back to white. I also quietly remind myself that it is just a thought - it's beyond my control and that i should just let the thought come and go. We only remember things that we have a strong emotional reaction to - be that good or bad. So, by practising and eventually accepting a thought and letting it pass through your mind and move onto the next thought you don't have as much of an emotional reaction and you are less likely to have that thought again. It does take a lot of hard work and it doesn't always work but it certainly lightens the load for me and allows me to focus on other things a little better. Hope this helps x