In a series of studies at Columbia University, it was found that people who successfully lost weight and kept it off had tried and failed a number of times before they finally succeeded.
When you change the way you eat or exercise, old habits tend to kick in and ruin your progress. To be successful, you first need to learn how to keep the changes and maintain them over time. Each attempt, even though it "failed," teaches you what you need to do to be successful in the future. In fact, the studies also showed that the more times you try to lose weight, the more likely your weight loss will be permanent.
So don't give up on yourself if you've tried and failed. Your very next attempt may be the one that succeeds.
The following chapters in Self-Help Stuff That Works will show you how to pick yourself up after a setback and continue. You don't need willpower, you need the ability to destroy the thoughts that suck away your soul; thoughts that take away your desire and motivation.