You can - because you believe that you can.
Your actions become your habits. Don't let yourself make a pattern of slipups or they'll become habits!
When you just don't think you can, keep going! Getting through those weak moments is how you become stronger.
Life is a challenge. Realize that and work, every day, to rise up and meet it.
The higher your energy level, the more efficient your body - and vice versa.
Life is too short to waste time harboring bad feelings about anyone. Forgive, forget, then move on!