A rap artist song called, "I am too light to quit."  

          Many a failure turns about
          When he might have won if he'd stuck it out
          So, stick to your task,
          Though the pace seems slow,
          For you may succeed with one more blow.
          You can never tell how close you are,
          It may be near when it seems afar
          So, stick to the fight and when you are hardest hit,
          It's when things seem worst that you must not quit.

     Sometimes, we are so close to a breakthrough, almost paper-thin close to a breakthrough, and we quit.