Paul says that if we walk in the Spirit, we will not sin ("fulfill/act out the lust of the flesh"). Why is that? Because there is no evil in the Holy Spirit. The Holy Spirit is God. So, if you are living your life harmoniously with the Holy Spirit, you will not sin. There is no point of harmony between the flesh and the Holy Spirit. There is no righteousness in the sin-nature. As Paul said, "Nothing good dwells in my flesh" (Romans 7:18).
The sin-nature is your enemy. The reason you, as a Christian, cannot do all the good things you would choose to do is because the sin-nature is obstructing you. The sin-nature is like scar tissue in a joint, binding up the muscles. Your conscience, when it is instructed by the Word of God, knows what is right. Your heart, which has been renewed by the Holy Spirit through your faith in Christ, instinctively desires to do what is right. But your sin-nature is still with you, gluing up the works.
Here is a scary thought: does this problem, this inner struggle, mean that you are forced back under the death-curse of the Law? When the flesh re-asserts itself in your life and you are still weak in your maturitty, does that mean you are going to hell? Not at all.
Those who are led by the Spirit are not under the law of death. Like sheep cared for by the shepherd, all Christians are under the care of the Holy Spirit. If the Holy Spirit is in your life, that means God has forgiven you of your sins! You would not have the Spirit of God if you were not redeemed from the curse of the law. The Spirit's presence in your life -- even as His presence creates that inner tension between conviction and performance -- His very presence should give you confidence that you are not under the law of sin and death. The curse of the law is gone, and the proof of it is that you have the Spirit of God in your life That's good news!
You need this confidence, so that you can move forward in your Christian life.
One of the flaws in the way I was taught as a young Christian was that my teachers confused the Law Covenant with "law" in the sense of moral commandment. Many of my teachers, including my Scofield Bible notes, taught me that Christians are not under any sort of moral commandments. Why? Because we aren't "under law." Because of this confused mingling of categories, my teachers watered down the moral teachings of the New Testament to the level of "guidelines. " They were allergic to the idea of moral law, and about had conniptions if you used the word "law", even though Jesus himself called his teachings "commandments" in the gospel of John. The formal term for what I was taught is antinomianism. All this antinomianism did was keep my muddled liberal conscience in a state of muddledness, and crippled my growing ability to discern right from wrong.
The apostle Paul was not an antinomian. In Galatians 5:13, he exhorts the Christians not to use their freedom from Moses' covenant as an excuse for sin. Instead, he orders us to serve one another in a spirit of love. Then Paul quotes Leviticus 19:18, "You shall love your neighbor as yourself." Christ fulfilled the Law for us legally, by dying on the cross for our sins. His fulfilling of the Law allowed us to receive the Holy Spirit. Then, after we receive the Spirit we have the power to fulfill the Law ethically, by loving our neighbors-in-the-faith.
The contrast to love is the dog-pack mentality (verse 15). I have seen members of congregations turn on other members in the spirit of a dog-pack. Even though they make excuses for it, you know it is the opposite of love.
But how can we keep the love commandment, which basically sums up all the New testament commandments into one sentence? We have no more natural power to love one another with God's love after salvation as before we became Christians. Instead, Paul says we must live our lives ("walk") in the power of God's Spirit. If we do that, then we will not carry out the desires of original sin, which Paul dubs "the flesh." There is nothing inherently evil about our physical bodies, and Christ will give us our physical bodies back in a glorious form when He returns. But our physical bodies represent the old human condition of still being the sinful children of Adam, so Paul signifies our lingering sinful thoughts and feelings with the word "flesh."
Every Christian is morally obligated to love, and to think, feel, and do all of that which springs out of love. Freedom from Moses' covenant is not freedom from love. In fact, it is the freedom to love! As long as the Law condemned us, the Spirit of God could not dwell in our hearts. Once Christ's blood is applied to our case, through faith, the Law is satisfied and the Spirit is free to move right on, at which time He immediately begins shedding forth God's love in our hearts.
You have the obligation to love, and you also have the power to love.