Willpower is a muscle, not a skill. A skill is the product of an action. A muscle possesses the capacity to produce of an action. Willpower is akin to a muscle in that when it is used, its energy reserves are depleted until it has had a chance to recover.
And like a muscle, willpower can be trained so that its total energy reserves are increased. Its capacity can expand through deliberate action.
“Sometimes it looks like people with great self-control aren’t working hard — but that’s because they’ve made it automatic. Their willpower occurs to them without them having to think about it.” -Angela Duckworth