I used to feel the same way. Are you exercising? You should try running a few days a week, maybe start by walking if you're not up to running just yet.
Try changing your diet if needed.. cut out the junk food and soda. Along with a better diet, try taking a multi-vitaman. I use and suggest
Centrum Performance.
Lastly, don't stay up way too late and get plenty of sleep. At least for me, I feel better if I sleep from midnight to 8am, than if I'm up to 4am and sleep till noon. Try to keep a regular sleep pattern too.
If you can do all of that, I can pretty much guarantee that you will feel better, and after a while you'll probably look better too!