Read any interview of a successful individual and you'll often find a common thread is persistence. They persist past the point when most people quit.
How can you gain the persistence edge?
Bob Parsons once told me "Success exists just beyond the point when the urge to quit is the strongest." If you life by that philosophy, you'll never quit.
Another analogy is the one you've probably is about the captain whose missed making it to land after a long journal. Despite being so close they could touch hit, fog hid it from view.