Echoing Juni and others, I take walks and get outside for some deep breaths of fresh air. Walking outside is where I get my best thinking done too. I've also had some psychotherapy over the past year and that has helped me to reframe negative thinking into positive thinking as soon as I recognize it.
For instance, I'll catch myself thinking, 'you are so stupid and never do anything right.' To which I'll stop and reason with myself, 'that is black-and-white thinking and doesn't even make any sense. you've been wrong before, and you'll be wrong again. that doesn't mean you don't do things right. you do plenty right..." etc. You get the point. It helps a lot to ask oneself questions and to reason within one's brain. We do each have that own personal power and we need to just empower ourselves to think positively and overcome some things and feelings.
That's not to say there aren't plenty of very real problems that need stronger assistance than one can do on her own. Getting help from others when needed is a very powerful thing to do too. Many people are too proud (or held back for some other reason) to do so.
It also helps me, and always has since I was in high school, to sit down and make what I call "Happy Thoughts Lists," which is basically just listing off the top of my head the things, people, sounds, tastes, memories, places, whatever in life that make me happy. It's also fun to keep the lists and go back and read them again.