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“Motivation” is the reason for which you choose to do something or behave a certain way. Having a lot of motivation is simply having a strong understanding of why it is you wish to do what you want to do.
“Willpower” is a combination of determination and discipline to do something or behave a certain way, despite having a clear understanding of the challenges involved with doing the action.
Summary: Motivation is having a purpose. Willpower is having the balls to do it.