Why Can't I Fall Asleep for Hours?

so i go to bed and then i cant fall asleep for about 3 – 4 hours.
ill just lay there and think about everything because i cant sleep.
i usually wake up around 12 lol
and i usually go to bed around 12 – 2
then itll be 2 o clock and i wont go to sleep until 4 or 5
its really frustrating because sometimes i will be tired
then ill stay up a bit longer doing whatever
ill still be tired then i go to bed and then im not tired !
is there any way of making myself fall asleep faster?
cause schools in a couple of days and i can never go to bed early
like at 10 or 11 i cant sleep at all.
and ill have to wake up around 7 30 or 8 for school
and i really dont know how im gonna do it lol

Suggestion:

Some light exercise at bedtime often helps you to relax, unwind and switch off but not strenuous exercise.

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