You can either tell yourself, ...

You can either tell yourself, 'It's too hard, I don't think I'll be able to do it,' or 'Yes, I can do this. Let me at it!' Only a fine line separates the two. But I'll tell you one thing: if you work like crazy, you'll come to display capabilities that you never knew you had--potentials that you always possessed, but never tapped.
 Josei Toda

Quotes from the same author

Philosophy must never be divorced from action. Unless it is put into practice, philosophy is nothing more than an intellectual game.
 Josei Toda