If you think you are beaten, you are.If you think you dare not, you don't. If you like to win but think you can't It is almost certain you won't.
If you think you'll lose, you're lost. For out of the world we find Success begins with a fellows' will. It's all in the state of mind.
If you think you are outclassed, you are. You've got to think high to rise; You've got to be sure of yourself before You can ever win a prize.
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.
~Walter D. Wintle
(There is disagreement as to where this poem originated. I discovered it a while back on the pages of Napoleon Hill's 1937 classic "Think and Grow Rich".)