Willpower definitely works, but it’s limited. There are impediments, such as fatigue, social influences, environmental factors, etc.
Willpower is ultimately determined by how important something is to you. There are certain situations in which people won’t give in. They’ll die before they’ll give in. Not that that’s always a good idea, but it’s willpower nonetheless.
The strength of one’s belief in a particular principle or outcome determines the strength of one’s will. All the more reason to get the beliefs right.