Your biggest challenge isn't someone else. It's the ache in your lungs and the burning in your legs, and the voice inside you that yells, 'Can't," but you don't listen. You just push harder, and then you hear the voice whisper, "Can." That's when you discover that the person you thought you were is no match for the person you really are."
-Unknown
Another:
A person who never fails, never does anything.
Another:
It's alright if you keep trying and keep failing. But strive to fail better each time.
Another:
Don't keep trying until you finally get it right. Go further. Keep trying until it is impossible to fail.
Another:
"Nothing in the world can take the place of persistence". -Calvin Coolidge. There is more to this quote that I don't necessarily believe to be true, but I do believe that persistence can often overcome innate talent, eventually.
And finally, and I'm probably paraphrasing the actual quote: Whether you think you can, or think you can't...you're right.
I've had several situations in my life where these mindsets have helped me.
Maybe they will help you too.