Willpower is insufficient

Willpower is the amount of mental energy (ie desire) and determination one has to do something difficult.  Willpower can be a powerful force.  When combined with perseverance it often results in impressive accomplishments. However it also varies from day to day, hour to hour, depending on factors such as personality, fatigue, calorie intake, sobriety, environment, experiences, etc.  Willpower grows stronger through training and discipline.  It will also weakens as failure takes it's toll.
 

LINK: Do you suffer from decision fatigue?
 

The point is that natural willpower has limits.  If we are relying on our own will to overcome lust and temptation we will inevitably fail.  We may last a week, a month, six months, sometimes even longer, but a time will come when we are vulnerable and sin, always waiting at the door (Genesis 4:7), will take advantage.

But aren't we supposed to have self-control? Self-control is similar to willpower, however there are key differences.  First, willpower is usually related to a desire to do or obtain something. Conversely, self-control is a restraining power, the connotation being that we are reigning in our passions and ruling them. The key difference lies in the source of the power.  When the power comes from within ourselves it is insufficient to overcome sin. When the power comes from God we can trust that we will persevere. 

Yet those who wait for the LORD will gain new strength; They will mount up with wings like eagles, They will run and not get tired, They will walk and not become weary.  Isaiah 40:31