I suddenly feel horrible about my current situation

Sometimes I feel like that too.. what I normally do is reassess my situation. I ask myself, why do I feel like this? What can I do to make my situation better? After I've answered those questions and realize that there's really nothing I can do at the moment, I try my best to accept it and do whatever I can to improve my mood. I just learned that dwelling too much on things that I can't control ain't really doing me any good.