These videos help me feel better when I'm down, but I recommend just finding something that'll make you laugh or distract you long enough so you forget about it, at least for a little bit.
http://www.youtube.com/watch?v=EABSk4JyB-0&feature=related
Possibly the cheeriest tune I've ever heard.
and
http://www.youtube.com/watch?v=yvapzt8FKLk
Don't know if it's your sense of humour but it makes me laugh.
From the sound of your post you do sound like you are tired. Unless you're planning on doing all you homework as a distraction tonight I suggest putting it in order of importance and maybe doing a little of it out of the way before you have a good nights rest - that way you won't be thinking about it before you go to sleep. Also, try to eat something that makes you feel full - it doesn't have to be sweet (cereal is quite good), because when you're depressed it's odd but you really do feel empty inside, so it's weird how much better you can feel once you've eaten.
One more thing! My friends used to use me as their agony aunt; I found telling someone (my mum) what they had told me (in confidence of course), made me feel better as I didn't have to keep it all pent up inside.
Hope you feel better soon. It's not nice feeling that way.