Sleep usually comes naturally to me, unless for some reason or other I'm totally worried about it. Then I have the kind of experience that Poet deVine describes...
My environment is ideally lights off, windows shut (pitch dark), television off, as many sensory avenues as possible cut off. The difficult thing to shut off is my brain.
Before I go to bed, I begin to worry---will I sleep well enough to perform on my exams? Of course, I think. Eventually I'll fall asleep.
The problem is that I start trying to fall asleep, which never works. Five minutes of laying with my eyes closed and I get impatient, and ask myself, why am I not asleep yet? Maybe I'm not comfortable, so I find another position. Should I try laying perfectly still for a while and hope I fall asleep or maybe would I have more luck in another position? So I find another position and hope it works out a bit better, but with no luck. I turn over my pillow because the other side is colder, and for some reason, I seem to be getting the sweats...
Then I start imagining things, just before sleeping, because for some reason I think they might help. I try to picture myself, say, somewhere I go normally---or, I try to imagine the sound of a familiar voice, preferably one I haven't heard in a long time, and maybe if I hear it clearly enough it will stimulate a dream. The main focus is to try and distract myself from the thought of falling asleep itself.
Of course, this method doesn't work. One of my exam days contained three exams, starting at 9 in the morning and ending at 9 in the evening. I got no sleep the night before, and I think I pushed the limit of caffeine for the human body in order to get through it...
I had another exam in the morning with no sleep, which wasn't nearly as bad, and my last exam finally let me sleep a bit the night before.
I used to have trouble sleeping with someone in the bed with me, like you say, Serenity---but lately I've gotten over that trouble. Have you mastered the art of spooning? The only problem is figuring out what to do with one arm while you're spooning a person, but once you've done that, you've got a potentially very comfortable sleeping position (though you can never be sure the other party feels the same way---especially if you tend to drool or snore).
Another sleeping position that's comfortable with another person, I find, is me laying on my back, with her cuddling my arm like a stuffed animal and sleeping on her side. I'm told it's strange I can actually sleep on my back, though, so this might not be so happy an alternative. A closer-contact version of this method is the same thing, but instead of having the arm cuddled in this way, putting my arm around her shoulders and holding her against me, while she uses my chest as a pillow.
This is maybe getting a bit personal, but I find it troubling you're unable to find a solution to sharing a bed and hoped I could share a little personal experience. It's awkward only at first, until you master it.