This is a great saying from the book Mind Gym:
If you think you are beaten, you are.
If you think that you dare not, you don't.
If you'd like to win, but think you can't, it's almost certain you won't.
If you think you'll lose, you've lost.
For out in the world you'll find sucess begins with a fellows will.
It's all in the state of mind.
Life's battles don't always go to the stronger or faster man; but sooner or later the man who wins is the man who thinks he can.