I can't fall asleep at all at night, not even for an hour. I wake up exhausted. But I can sleep fine during the day. I don't understand, and I'm so tired.

Are you sleeping during the day?
You sound like you have your days and nights mixed up. The best way to fix that is to not allow yourself to sleep during the day and keep really busy so that you are tired by night. Do that at least until you can sleep at night.
If you don't work and exercise during the day that is one sure way to end up getting your days and nights mixed up.

Also you could have an imbalance like with your serotonin or adrenaline but then, I think, you probably wouldn't even be able to sleep during the day either.

There could be thoundsands of reasons for this, some medical and some not.

N0help4u pretty much said it already, but the simplest way to rule out a non-medical condition is, everyday, even on the weekends, wake up at the same time, go to bed at the same time, and never nap during the day. After 2-6 weeks of this, it'll reset your circadian rhythm and you'll be human again. Also, be conscious of your habits when you sleep, like clinching teeth, constantly looking at the clock, how evenly your breathing.

This is what worked for me.