Romans 7:15,18 (NKJV)
For what I am doing, I do not understand. For what I will to do, that I do not practice; but what I hate, that I do.
For I know that in me (that is, in my flesh) nothing good dwells; for to will is present with me, but how to perform what is good I do not find.
Do you ever feel that way? Can't seem to get it right and messing things up over and over again? Yeah, me too. And the worst thing about it is that you can see it happening when you are doing it. Paul is describing his own experience.
For a Christian, there are two natures at war with each other. There is the fleshly nature and then there is the spiritual nature. When speaking of the flesh in this context, it is not meant to be simply the body but rather, the flesh represents the desires to illicitly satisfy physical longings. We know that there are areas of the brain that are wired up to powerful pleasure centers of the body like eating and sexual activity. For the Christian, there is another part of your mind that is wired to seek spiritual things. That is the part that is "born again". You can see how these would often, but not always, be at odds with each other.
Which side wins? In a battle it is the strongest that wins the battle. Which army is the strongest? The one that is the most, trained, practiced, well-fed, and obedient to its master. Feed the spirit, starve the flesh; obey the spirit, deny the flesh.
Romans 7:24-25 (NKJV)
O wretched man that I am! Who will deliver me from this body of death? I thank God--through Jesus Christ our Lord!
So then, with the mind I myself serve the law of God, but with the flesh the law of sin.