This is one of my favorite positive thinking quotes:
If you think you are beaten you are;
If you think you dare not, you don't;
If you want to win but think you can't;
It's almost a cinch you won't.
If you think you'll lose you're lost;
For out of the world we find
Success begins with a fellow's will;
It's all in a state of mind.
Life's battles don't always go
To the stronger and faster man,
But sooner or later the man who wins
Is the man who thinks he can.