One dictionary defines willpower as: control of one's impulses and actions; self-control. 
Today, we equate willpower to things like smoking, drinking and over-eating, although, there are so many things we can tie willpower to.
Exercise, work, and love among them. 
Everyone has the capacity to harness willpower, it is something that you just have to put into use.
A smoker may try to quit many times, each time putting willpower into play. It is when the willpower is not being used that the craving takes over.
Similarly an alcoholic or drug user may stop using for a time letting willpower become the driving force. Once the willpower is forgotten the consumption returns. 
What keeps it strong in some, but weak in others? 
Some say it is a resolved mind, while others say it is the ability to resist. 
Whatever it may be, we see it everyday even if we are not aware. 
The person who chooses to jog or take the bicycle to work instead of the car. 
The person who makes a lunch instead of going to the restaurant. 
We may never know the reason why they made these choices, but you can be sure willpower was involved.
Next time you feel the urge, try to remember these wise words from author Robert McKee "Most of life’s actions are within our reach, but decisions take willpower."