The sin nature is our human nature that is bent towards doing wrong that we received from Adam, which was a result of his determination to violate God's law in the garden.
His sin brought physical death:
Because of this, just as through one man the sin entered the world and through the sin the death, also thus unto all men the death passed, on the basis that all sinned. - Romans 5:12
His trespass produced spiritual separation from God:
For since in the trespass of the one [man] the death reigned through the one [man], by how much more the ones receiving the abundance of the grace and the gift of righteousness in life will reign through the one Jesus Christ. - Romans 5:17
When a person is saved they become part of a new creation. This is a new man in which Christ is the head and the Church is the body. Being a part of this new man releases us from the condemnation that was a result of Adam's trespass and sin and imputes Christ's righteousness to us. Having been washed and regenerated, we now have a new nature. One that is connected to God in our spirit. However, because we do not yet have our resurrected bodies we struggle with the sin nature that resides within the flesh.
For we intuitively know that the law is logical, but I, on my part, am fleshly , having been sold under the sin [nature]. For that which I (old nature) work out, I (new nature) do not experientially know, for that which I (new nature) do not desirously will, this I (old nature) practice, but that which I (new nature) am indifferent to, this I (old nature) do. But since that which I (new nature) do not desirously will, this I (old nature) do, I (new nature) together say with the law that [the law] is proper. But now I (new nature), on my part, no longer work it out for myself, but the dwelling in me sin [nature]. For I (new nature) intuitively know that dwelling in me, that is in my flesh, is no good. For the desirous will is present with me (new nature), but the working out of the proper, it is not found. For not that which I (new nature) desirously will, to do good, but that bad which I (new nature) do not desirously will, this I (old nature) practice. But since that which I (new nature) do not desirously will, this I (old nature) do, it is no longer I (new nature) working out it, but the sin [nature] residing in me. Then I find the law to desirous will in me to do the proper, that in me the bad is present. - Romans 7:14-21
and you will know the truth and the truth will set you free ... Jesus answered them, "Truly, truly, I say to you that all the ones doing the sin are slaves to the sin - John 8:32,34
The truth is the doctrine by which we have victory over the sin nature. That part of us we struggle with every time we desire to do what is good.
We start out with knowledge based upon the truth (seeing things as they really are). Because we are immersed into Christ's death and resurrection, that is, God puts to our account Christ's death and resurrection, the sin nature has been rendered ineffective.
experientially knowing this, that our old man was co-crucified in order that the body of the sin [nature] should be rendered ineffective, so that we are no longer slaves to the sin [nature]. - Romans 6:6
The next step is logical because we have to take God at His Word in regards to the truth of our immersion into Christ and how it impacts our lives now. In Christ, we have died to the sin nature and are raised to walk in newness of life.
Thus also, you should reckon yourselves to be dead. On the one hand to be [dead] to the sin [nature], on the other hand living ones to God in Christ Jesus our Lord. - Romans 6:11
Since we are free from the rule of the sin nature, we need to start acting like it and stop yielding our members to its desires.
Neither present your members instruments of unrighteousness to the sin [nature], but present yourselves to God as out from the dead being living ones, and your members instruments of righteousness to God. - Romans 6:13
Having victory over the sin nature is not about saying no, but about doing what is right. As we yield our members to the desires we have to do good, the Holy Spirit influences us so that even though we struggle with the desires of the sin nature, we are able to overcome them and live a life apart from sin.
But I say, walk by the Spirit and you will never fulfill the desires of the flesh. For the flesh desires against the Spirit and the Spirit against the flesh. For these are contrary to one another in order that you should not do the things you desire - Galatians 5:16-17
Trying to use any negative law against the sin nature does the exact opposite of what we would expect. Law is good and logical. By the Mosaic Law humans can see how to be righteous. However, it is also by this same law that we learn about our nature and how sinful it is.
Therefore, what will we say? Is the law sin? May it never come to be! But the sin [nature] I would not have experientially known except through law, for I would not intuitively known the covetousness, except the law said, “Do not covet.” - Romans 7:7
Law actually gives the sin nature strength so that it overcomes us and puts us back into slavery to its desires.
But the sin [nature] taking the occasion through the commandment, worked out in me all covetousness, for apart from law sin is dead. - Romans 7:8
The sting of death is sin, and the inherit ability of sin is the law - 1 Corinthians 15:56
When we yield our members to what is right, and stop using them as instruments of unrighteousness, we free ourselves from the rule of the sin nature because we are taking God at His Word and showing that faith through our actions.
The commandments for a Christian are:
Love other Christians, John 13:34; 1 John 3:23
Abide in Christ, John 15:1-4; 1 John 3:24
Love does not produce anything contrary to law, Galatians 5:22-23; therefore, we do not need law to guide us.
How do we walk by love?
Examine your actions before you do them.
Are they seeking the best for the other person?
Do they violate any aspect of God's desirous will for your life?
Will they be rooted in taking God at His Word - living out from faith.
If not, remind yourself who you are in Christ and look for doing what is good in whatever situation you are in (Know - Reckon - Yield).
When we catch ourselves using a "do not", we need to convert that to a "do". We do not live by law, that is, our lives are not governed by law, so we need to focus on what we should be doing, not what we are not to do. Focusing on "do nots" gives the sin nature strength, whereas focusing on what to do makes it impossible for the sin nature to produce what it wants.