Therefore, brothers and sisters, we have an obligation—but it is not to the flesh, to live according to it. For if you live according to the flesh, you will die; but if by the Spirit you put to death the misdeeds of the body, you will live. (Rom. 8:12-13; NIV)
Pregnant women sometimes get cravings. There are funny stories out there, like the lady with a craving for butter who, while on the phone with her husband, dropped her cell phone into a container of Country Crock.
Doctors really aren't sure why this happens to some women, but the theory is that the body has some nutritional need and projects it to the conscious as a specific (and sometimes highly peculiar) food. What I know is that I get cravings for food also, and not always the hour before dinner. It sometimes happens when I'm worried or unhappy, and for whatever reason, the craving I get is for something that isn't very healthy, like Pop Tarts. If I'm really stressed I might put a second batch in the toaster, even though I'm not very hungry after the first.
We do have hormonal triggers that cause urges. Eating is a good example, because when you're really hungry, it's hard to control yourself. But it applies to other things as well, and the urge isn't always satisfied by something positive. We might have sexual urges and no spouse to fulfill them, or loneliness with a desperate urge to be cared for and affirmed. We might have anxiety or emotional pain and no method to offload it except prescription medications or a numbing series of alcoholic drinks. We might have doubts about our value and engage in self-destructive behavior to end everything, to live on the edge to feel more alive, or to seek out something with one of these as our end-goal.
On some level, these urges are all normal and can be met in healthy ways, in Christ. As Christians, we have the indwelling Holy Spirit who can lead us to a solution that is within God's will or, if no immediate solution presents itself, gives us the strength to resist the urge. Some of these urges are powerful but, when resisted, will almost immediately fade. If you don't know Christ, you can still resist the urge but you don't have the presence of God to help you. Inevitably, your values may change so that you can meet your needs without harboring guilt. The world around us fosters values to make that possible.
For a Christian, there is a counter-force within you. It is the Spirit of God, and the Spirit empowers you and transforms you, if you will obey the Lord and seek his will. The Spirit directs you on the path of holiness. But it's still not easy. It's a battle, in fact. Paul talks about it throughout Romans 7 and the first part of Romans 8. His point, in those verses, is that simply knowing God's will (i.e., the Law) isn't enough. You need the Spirit, and you need to yield to him. But it's a struggle.
The phrase "sinful nature" does not occur in the New Testament (in the original Greek). Some modern translations, including the NIV, will sometimes (but not always) translate the word "flesh" this way. If you see "sinful nature" in Romans 7, substitute the word "flesh." I think that will be more helpful in understanding the struggle you face against your tendency to sin. If you have a "sinful nature" it almost sounds like it's natural for you to sin, and if it is, that you have an excuse not to stop.
The term here is "flesh," and I think Paul uses this word, in nearly every case, to refer to the body. If you understand it that way, his instruction makes sense. When you become a Christian, you don't get a new body. It's the same old body, same old brain, with its biological urges and learned patterns of disobedience (i.e., sinful habits). You still have your old memories, burnt into your neuropathways. Your body has been trained to respond to them. Your body is not neutral in the struggle to avoid sin. All of this changes at the resurrection, but right now, you're stuck with that part of you. If you look carefully at Rom. 8:12-13, you will see that Paul puts "live according to the flesh" in parallel with "misdeeds of the body." He's just rephrasing the first idea so that it doesn't sound repetitive. These are two ways of referring to the same thing.
We have the Holy Spirit to transform and empower us to achieve victory over the flesh, but we are not puppets. Victory over our habits and urges comes because we're mentally engaged in the battle. Since it's a battle against self, a better analogy is a race, where you fight your urge to stop running.
Do you not know that in a race all the runners run, but only one gets the prize? Run in such a way as to get the prize. Everyone who competes in the games goes into strict training. They do it to get a crown that will not last, but we do it to get a crown that will last forever. Therefore I do not run like someone running aimlessly; I do not fight like a boxer beating the air. No, I strike a blow to my body and make it my slave so that after I have preached to others, I myself will not be disqualified for the prize. (1 Cor. 9:24-27; NIV)
When he says, "strike a blow to my body and make it my slave," it's like slapping your own hand as it reaches for the Pop Tart. Or if you're about to cross the line sexually, dumping cold water on your head. Or if it's some addiction, calling a friend who will talk you down. Whatever it takes. Sometimes it's just help for that brief moment that allows you to get in sync with the Spirit, to bring you to your knees in prayer.
The struggle gets easier as you stop giving in. You train your flesh (rehabituate yourself) to following the Spirit. That's holiness, and it's an incremental process. But you only get there by taking that first step. And then the next, and onward. Until the day that you hold the crown that will last forever.