Best of luck dude. Sometimes its just the mood that you HAVE to experience to understand what happy is. If you understand happy, you must understand sad. Same with the hot/cold concept.

Dude, I get that way all the time but then evaluate why.
1.) I hate my job and that truly builds up on me and I leave the day on a sour note majority of the time.
2.) Not getting sleep due to school, work, gym....
3.) Getting stronger but not building the mass I want.

HOWEVER, if you step back and look at it, all those things are positive. Im paying my own way through school, I have a decent job with benefits and Im in pretty damn good shape. Its just how you look at it dude. Take some time to step outside the box and maybe change your thought process. A guy at my work told me once and I'll never forget this: "You can think about something in two different ways...either positive or negative, so why not make it a positive one?"

Best of luck dude. If all else fails, post nudes of ex-girlfriends in spite.