Where does willpower come from?

I’m forever struggling with food and my weight (this isn’t about this specifically), and realize a huge part of my problem is willpower.

Whether you’re trying to eat right, exercise, study regularly for a class, volunteer, or accomplish some other goal, it’s more difficult for some to muster up the willpower to resist giving in to distractions and temptations.

Where do you find this “willpower?” Is it inherited, taught, discovered, earned, etc? How can you improve it? How do you fight the id? Does everyone have it and some people are just lazy and greedy? Is there just no excuse for this weakness?