I've been asking myself the same question lately. And the answer, at least for me, is that sometimes you can't always stay positive. It's okay to break down. I mean, don't get me wrong, its good to try and always be positive because it will make you, and everyone else around you happier. For a while I was sort of stuck in this "rut" if that makes any sense, where I was unhappy for a long time, and I thought that I had nothing to look forward to. But then I started to pay attention to the small things, like how it felt to take a nap or a nice long shower after a long day at school, or how nice it feels to hang around family and friends that you love. So when I'm feeling negative I think about how those things comfort me and make me happy, because I know that when the thing that I'm dealing with is over I can go and do those things. When I'm not able to think of anything positive in a situation that I can't get out of, I always think "It's not going to be like this FOREVER, and even if I feel like it's the end of the world, it could be worse in some way, no matter what."
I hope that this helped at least a little bit. :) xx