I stumbled over this on one of the crossfit websites today:

When an athlete says "can't", a coach hears "won't".  To unlock your true potential you need to remove "can't" from your vocabulary.  Or at the very least, be honest with yourself and what you truly mean when you say it.  Try replacing "can't" with "I don't want to try because I might fail" and see whether your attitude matches your goals.