Destruction of Negative Thinking

Someone once told me that “what you focus on grows.” I believe it actually means that whatever your my mind focuses on will grow. If I focus on success, I’d definitely have success.
One thing we must know is that, the mind is no respecter of persons. So if your mind is focused on the negative then you get a negative reality. Take for instance, Defeat. If you fill your mind with defeat, defeat becomes your focus and it will eventually reflect outwardly in everything you do. Like Napoleon Hill said,

"Like the wind which carries one ship east,and another west, the law of autosuggestion will lift you up or pull you down, according to the way you set sails of thought."

Here's a poem that better illustrates what i'm talking about:

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 certainyou won't.

If you think you'll lose, you're lost
For out in the world we find,
Success begins with a fellow's will-
Its 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!