You are in control of your thoughts. I know that it feels like "you can't help it" sometimes, but you can.
I took a tip from someone here on SR (I wish I could remember who!) and wore a big rubber band around my wrist. Whenever I started obsessing, worrying, turning things over and over in my head, and generally making myself miserable, I'd give that band a big POP!
Bring myself back into Now. Into reality.
He's only taking up space in your head cause you haven't figured out how to kick him out yet. You'll figure it out.
I think that Denny's suggestion is a great idea, too. Nothing gets me out of my head quicker than doing something that needs doing, for someone who really needs it done.
Good luck, genrs.
-TC