I, too, struggle with the Negative Nancy living in my head.
Sometimes when I need encouragement, I look at my life now as compared to where it was 5 or even 2 years ago. I relax on my couch, and look around my house and tell myself "I worked for all this stuff, and I can AFFORD all of this! Some people can't." Now, I don't have a huge, glamorous house by any means, but I have one!
If that doesn't work, sometimes a hot bath and a nap make me feel better.