Impossible to fall asleep!

I'm a 19 year old girl. Since I was about 12, I've been suffering from insomnia. I have fallen asleep before 3am probably only about 50 times in the past 7 years. Sometimes I cannot fall asleep until 6 or 7am and I wake up several times each night. This made school very difficult for me (coupled with my depression). When I was 16 I began taking various sleep medications my doctor prescribed. The pills either didn't effect me at all, regardless of the dose; or knocked me out and I couldn't be woken up fully for 12+ hours after I took it. I have managed to (through depriving myself of ANY sleep for a night) fall asleep before midnight on several occasions. However, within a week, I'm right back to lying awake in bed all night.
Now that I'm no longer in school, finding and keeping a job has been very difficult. Waking up before noon when I'm not asleep until 6 in the morning is not very easy at all, and when I don't start looking for work until 1 or 2 in the afternoon, most managers have left for the day. And on the occasions when I've had jobs, the longer I have them the worse my sleeping gets. I end up getting to the point where I won't sleep more than maybe 4 or 5 hours in a week.

I was wondering if anyone had a suggestion as to how I can manage my insomnia to improve my life?