This quote applies to life as much as it does to sports;

Do you believe you're a starter or a benchwarmer?
Do you believe you're an all-star or an also-ran?
If the answers to these questions are the latter,
your play on the field will reflect it.
But when you've learned to shut off outside influences
and believe in yourself,
there's no telling how good a player you can be.
That's because you have the mental edge.
-Rod Carew