Answer: Believe that you can! Believe willpower is a muscle you can strengthen with exercise.
Recent findings by Carol Dweck and her colleagues challenge current thinking that willpower is exhaustible and fixed. She found that the subjects who believed that exercising their willpower strengthens it rather than depletes it actually increased their willpower. Their mindset was the key.
Willpower: It’s in Your Head
Willpower can be an unlimited resource, study says
Here is the original scientific paper by Job, Dweck & Walton: