I'm the exact opposite (usually) from my girlfriend. It usually takes me about 30-45 mins to fall asleep, but if my girlfriend starts thinking about not being able to fall asleep, she'll be up all night. I think that she just psyches herself out of being able to sleep though, plus she's a light sleeper.
I used to have major troubles sleeping, and it sounds kinda like what happens to you CHEWIE - I used to be thinking about too many things and I couldn't relax enough to sleep; I would be too busy thinking about different stuff. I'm not sure what I did, but since college I have (usually) been able to just fall asleep without much effort. I still think about a lot of stuff before going to sleep, but I try to keep it either unfocused (try not to think about something for very long) or very detailed (step by step walkthrough of how to do an upcoming custom). I try to avoid the middle ground because that seems to be when I have trouble sleeping...