I know what you mean. When I'm stressed out my brain focus's on a single stupid thought and I can't sleep. It isn't necessarily what I'm stressed about most of the time. Normally I come up with stories in my head and daydream which eventually allows me to fall a sleep, but when I'm stuck on a single thought I can't do this. I hate it -_- But yeah like I said I find the best way to stop this is to daydream about something or read before bed.