You need something else to occupy your mind.
Someone tried this mental exercise with me: he said, “Don’t think of a big white elephant. Don’t think of a big white elephant with floppy ears.” and repeated it several times. After a while, you will start to think of the thing you are trying not to think about. Why? Because you are concentrating about it.
The solution is to actively think about something else (or to concentrate about not thinking about something else – like white elephants 🙂