Pastor peached on it this morning. The primary reason that we fail is because (aside from our sin nature) we allow other things to become our focus. Peter sank in the water when he saw the wind and the waves. This is why we are to make a practice of prayer without ceasing. And this
Finally, brethren, whatever is true, whatever is honorable, whatever is right, whatever is pure, whatever is lovely, whatever is of good repute, if there is any excellence and if anything worthy of praise, dwell on these things.
PHILIPPIANS 4:8 NASB
This is our combat against besetting sin.