Difficult to fall asleep despite preparations?

First things first, please excuse me if I spell some things wrong. Im not from an English speaking country.
Now, the problem. Often when I try to sleep I find myself unable to actually do just that. I lay in bed, completely relaxed, in a comfortable position, lights out, everything I can think of to give myself a good sleep. About an hour before I go to bed I turn off my computer, put away my phone and ipad and do nothing but get ready for bed. Still, I can lay in bed for a very long time before I actually drift away to dreamland. One time it even took me as long as two hours before I managed to fall asleep. Is there something I'm doing wrong? Is it some kind of disorder? Doe's anyone know what I can do to fix it? Im still in school and the grades I get this year are crucial for my future studies. I really don't want to screw up because my eyelids fail to remain closed.

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