Well I don't know if you can. I've had this problem a few times. My mind just goes off and it's ridiculous. Doesn't happen as much now as when I was on my meds for ADD, that was insane. But yeah even when you stop thinking about deep shit it tends to pop back into your head again and thus the viscious cycle continues.

The only way to solve this would be to silence your mind. And seeing as how that's easier said than done I would suggest getting into meditation perhaps? That's a pretty good way to clear the mind so they say.