There are very few things as frustrating as doing something and then looking at yourself with disgust and amazement because you did the very thing that you have so often said should never been done.
This is what Paul initially felt. But then he came to an understanding of what the reason problem was. It was carnality in his nature. And no matter how hard he tried, this carnal tendency stayed with him.
It might do us well to remember that when we were saved, we were made new men in Christ, partakers of his divine nature, citizens of heaven and children of God. But our flesh was not redeemed. It is still as wicked as it has ever been.