This is from Napoleon Hill's classic, "Think and Grow Rich!". Although the principles are related to wealth creation, the book is applicable to any specific goal. This verse comes from the chapter "Self-Confidence".

If you think you are beaten, you are;

If you think you dare not, you don't;

If you like to win, but you think you can't

It is almost certain you won't.

If you think you'll lose you've lost,

For out of the world we find

Success begins with a fellow's 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 soon or late the man who wins

Is the man who thinks he can.