Live up to Your Values
The single biggest factor of your self-esteem is your own values. And your confidence is a result of living up to those high standards.
Cultivate the personal integrity to live up to your own high standards.
Personal integrity in all aspects of life is essential - in sticking to your diet, pushing yourself at the gym, studying for the next exam, or doing whatever it takes to meet your customer's expectations.
Don't let yourself slide. It is a slippery slope when you start cutting corner, "just this once." Soon enough, without even realizing it, your high values are full of holes and your integrity is a joke.
Hold on to your integrity and stick to your values.