I've written several posts about this in my other blog, mostly when I needed something to do really badly, i.e. when I just couldn't sleep.
Apparently, you're experiencing 'difficulties in falling asleep' if it takes you more than 15 minutes. The times when I fall asleep in 15 minutes have been very, very rare. Almost non-existent. For me, it's usually an hour or two at least.
The trouble with not falling asleep, is that if I just lay there tossing and turning, my head will fill up with all the things I'm worrying about. And these things tend to grow in the dark. So then there's no way I'll fall asleep. Also, I'm usually too tired to do anything else, like read a book, or do some work, but apparently not tired enough to sleep. The logic of that still escapes me...
If I could at least be productive, it wouldn't be quite so bad, but spending hours on end just lying in bed, staring at the ceiling and thinking about everything that's wrong in your life is just so very, very boring. And probably not great for your mental health.