how much do you sleep at night ? do you exercise ? what type of foods do you eat ?
your going to bed to late. your quite young and your body does need more sleep for the growing thats going on... Sounds daft, but it is also the perfect time to start bed time rituals, if you have a bath before bed it'l prepare your body to relax, you'll be able to fall asleep earlier and wake up at the same time feeling more refreshed, naturally your getting about the right hours of sleep, they're just not consecutive. :)
Try doing sports, like pool or bicycle, or jogging, once-twice a week, our physics related to mental condition a lot, u will feel urself encouraged for more
Heard of Chronic Fatigue?
It could be many things, but might be worth speaking to your doctor about it.