Trying to stop thinking of something can be hard. A story from NewScientist.

“It works something like this. Say we’ve given up chocolate or cigarettes, and want to block all thoughts of them. We do this by filling our conscious mind with distracting thoughts – anything but chocolate or cigarettes. At the same time, though, our unconscious mind remains alert for any signs of the unwanted thought, the better to help us chase it away. “Some part of the mind has to know what it is we don’t want to think about and to monitor for that,”