God this is a HORRID bit of OCD. It was a lot worse when I was about 18-19. I wouldn't sleep at all because I'd stay up all night organizing thoughts and what have you. The way I got a bit better was by making a health ritual before bed time. Like shutting off all of my electronics (and I never did it in a specific order. Which was really hard at first), I would allow myself 20 minutes of reading then 10 minutes of "thinking time" when I finally settled into bed, then I'd make myself stop so I could sleep. It took a bit, but it helped a lot.