Trite answer here - try meditation. Force yourself to take a regular (daily) space in your life where you sit down specifically to empty your mind of all thoughts. By doing this you may find that your mind starts to respond to the stimulus (lack of stimulus?) and forge new pathways to stop your pattern of destructive thinking. It's worked for me, and may well do the trick for you.
The practice is about concentration and mindfulness[0]. It can calm your mind up to a degree when there are little thoughts present but you can't force yourself into this state daily, especially in the first years of practicing.
[0]: http://www.vipassana.com/meditation/mindfulness_in_plain_eng...