well I wake up at 7am dead tired. so shouldn't my body want to fall asleep at such a time that it will be rested when I wake up?
Sometimes I'm tired and all but still can't sleep. Too many things going through my mind I guess. It's been this way my entire life.
There can be an adjustment period where you're not getting enough sleep for a while before your circadian rhythm sorts itself. You have to be consistent. Not in when you sleep, but when you wake up. If you already are, well, it's probably something else.
There are a lot of different things you can try to to improve your sleeping habits. I'd say look to your diet and nutrition first, but yeah.

I assume you know to be getting enough zinc and magnesium, and maybe some melatonin if that helps.
What do you do in the 90-120 minutes before bed? You want to be doing something relaxing and/or boring.
Do you sleep in a warm room? (Cool and lots of blankets is better)
The way I know I'm ready for bed is I read, and when I start to lose my concentration and feel a sort of dull heaviness in my head, it's sleepy time.
One thing I think may be very helpful for you is to write it all down. All the stuff going through your mind, I mean. Maybe make a journal, or a "to do" list, or even set goals, to clear your head of concerns and/or worries. (Then address them the next day, once you are rested).
Anyway, there are lots of resources out there, check some of them out. Sleep is vital!