Doubt.

But seeing the wind, he became frightened, and beginning to sink, he cried out, “Lord, save me!” Immediately Jesus stretched out His hand and took hold of him, and said to him, “You of little faith, why did you doubt? -Matthew 14:30-32

Doubt is one of Satan's favorite plans of attack. In the Garden of Eden He created doubt in Eve's mind by questioning God's instructions (Genesis 3:1).  He will use every opportunity to create doubt in the life of the believer.  James 1:5-8 talks about a man who doubts is "like the surf of the sea, driven and tossed by the wind."

This is especially true when we have turned aside and are walking in sexual immorality.  Peter took his eyes off of Christ and looked at his surroundings.  I strongly doubt that your spiritual life is thriving while you continue to feed the lust.  In order to act out our sin we must put God out of our minds and silence our consciences.  Even our attempts at fighting lust are weak and fail because they are based on our own willpower. Furthermore, we lack the commitment to fight and overcome our sin because in truth we love it and don't want to stop. 

We all have doubts slip into our minds at times.  This is normal.  But when we allow doubts to shake our faith and create fear we are essentially telling God that we do not believe He has the power to save us!  Worse yet, we try to tell HIM how He needs to save us!

In order to begin to walk we must be set on a foundation of faith in God and His promises.  We must fix our eyes on Christ and ignore all the doubts and worries that try to drag us down and keep us immobilized. (Rise up and walk)  We must purpose in our hearts to be obedient no matter what the cost.