The secret of success is persistence. But what happens when you hit a setback and feel defeated? When you conclude your goal is impossible, it takes a tremendous amount of willpower to persist.
But when you feel defeated, you have almost surely concluded something that is false: Nobody will ever want my product, I'm just a loser, Everything I touch turns to crap, etc. When a setback occurs, it brings you down. When people are down, their thoughts become overly negative, distorted, and exaggerated.
Learn to seek out the mistakes in your thinking and correct them.
Don't try to persist by forcing yourself to continue even when you feel defeated. Learn to poke holes in your defeated conclusions and you will persist naturally. Without willpower.
Read more: Pillar Of Strength
and Positive Thinking: The Next Generation