I was going through a similar problem a while back, and this is what the doctor said:
Go back to your regular schedule as if school has started. This means waking up at 6. Do physical activity throughout the day, some in the morning, and then some in the afternoon. Try not to fall asleep between 2-4, try your best to keep awake. Then try to sleep at 9-10ish. If you are having trouble getting to sleep, then read a book until you fall asleep. Make sure the room is dark and quiet. If you wake up in the middle of the night, pick the book back up and read it. The continue the cycle.
It might work for you. If it doesn't, my doctor suggested an allergy medicine that will help you fall asleep. If not that, then try an over the counter sleep medicine.
My doctor also said to stay away from caffeine and chocolate past 3 pm
because it won't let you go to sleep at that 9-10 deadline. So make sure you avoid that.
If this doesn't help you, then you should really go to your doctors again, explain what you have tried, and they might be able to help you from there.
I hope this helped, and I hope you will be able to sleep better.