First of all, there are chemicals in your brain associated with happiness, and if those are out of whack, it makes it almost impossible to 'just get over it and grab life by the nuts.'
Get that checked out first, and if you're currently on anti-depressants and feel they aren't working for you, keep trying. It's a delicate balancing act taking place in the most misunderstood and complex organ of your body.
As far as behaviour, all I can tell you is what worked for me. Get out. Get out of your house, your apartment, wherever you are, and do things. Your body will protest. Your mind will try to enforce laziness, because we're naturally programmed to resist true change. Find a friend you trust who will get out alongside you and go hiking, or volunteer at an animal shelter.
Oh, and write. I had a terrible childhood, and the only thing that kept me sane through it and my early twenties was writing in a fantasy world I'd been concocting in my mind since I was ten. Knowing that unhappiness is a cycle, I just sort of poured it all onto my fictional characters rather than the people around me. The result is something like Game of Thrones in space, but whether that's good or bad, it worked.
On a final note, make sure the real life people you surround yourself with aren't dragging you down, morally, financially, or emotionally. It's better to have a handful of good dependable friends than a hundred crappy ones.
