Get out there and do what you love!
There are a million reasons why you can't. Focus on the few reasons why you can.
Be unrelenting. If you don't believe, then who will?
I try to think about positive things - how great my form is, how my arms are swinging, my breathing, how loud people are cheering. My sports psychologist taught me there are a million things telling you you can't keep going, but if you find the things that say you can, you're golden.
Once you make the decision that you will not fail, the heart and the body will follow.
Never underestimate the power that one good workout can have on your mind. Keeping the dream alive is half the battle.