I think this is coming from the Henry Ford quote, which is "Whether you think you can, or you think you can't--you're right."
This quote basically means that what you think you can do is all in your mind and how you perceive yourself. if you think you can do something, then you'll be able to do it. If you think you can't do something, then you won't be able to, because you have already given up in your mind.
