Although I often come off pissed off(I mean, I am going to be a cop), I suppose it's just a persona. I'm a truly happy and blessed person - I can't ask for much more in my life. I make it a point to not let the little things bother me, and I make little deals of big deals to stay calm. I live by the motto "stay positive" - it's easy to be brought down by situations and circumstances, but if you can take the time to take a few steps back, look at things more objectively, and realize that this situation will pass and everything will be okay, it seriously helps with staying relaxed and happy.
Next time you're feeling down, get out a piece of paper(or textedit document) and write down everything you have(not just material things, but friends, family, love, etc) and seriously stop and look at it...