Q. Could you please define “sinful nature”? I am becoming more and more aware of my shortcomings and character flaws and am attempting to correct these behaviors. (I didn’t realize I was so sassy.) Are these flaws (bitterness, selfishness, directed by self-will, etc.) and defects a part of my “sinful nature”? Thank you.
First, let me say that you should actually be very encouraged by the way you’re realizing that you’re sassy, bitter, selfish, etc. (along with the rest of us who need God’s grace and mercy each day). If we have a growing awareness of the sin in our lives, this is actually one evidence that the Holy Spirit is at work within us to make us more like Christ, and so this is also one grounds for our assurance of salvation. So be encouraged (even if paradoxically)!
As for your specific question, in recent years Bible scholars and translators have been reaching a new perspective about what the “sinful nature” actually is. Previously it was held that when a person trusted Christ for salvation, this gave them a “new nature” or “redeemed nature,” but at the same time, they still had a “sinful nature.” This was considered to be a part of them that could continue to lead them into sin.
In other words, the believer was considered to have a double nature, and so growth in Christ-likeness was considered to be a matter of strengthening the redeemed nature so that its influence would be greater than that of the sinful nature. The example was used of the Inuit man who’d become a follower of Christ and said that there were “two sled dogs” fighting with him. When he was asked, “Which one wins?” he replied, “The one that I feed.”
But more recently, especially through comprehensive studies of Paul’s teaching on the Holy Spirit such as Gordon Fee’s massive volume God’s Empowering Presence, a new view has been coming into favor. The phrase “sinful nature” was formerly used in versions such as the NIV to translate the word sarx in Paul’s writings.* But that word simply means “flesh.” Many times it’s used to describe the human body, for example, when Paul says that Jesus “appeared in the flesh” (that is, he became human). But sarx can also refer to a spiritual force or influence. However, it’s now being recognized that this is not a force inside of us that’s part of us, but rather a force outside of us that tries to make us conform to a certain way of life.
Specifically, the “flesh” tries to make us conform to the way of life that corresponds to this “present evil age,” when God’s authority is not acknowledged and so people are “directed by self-will,” as you aptly put it. To live by the Spirit rather than by the flesh is instead to follow the way of life that corresponds to the “age to come,” when God’s authority will be universally acknowledged and honored, so that people will act as Christ did, in a way that’s loving and considerate towards others, not thinking of themselves first. There’s actually an overlap between the two ages, and we’re living in that overlap now, which is why we can choose to conform our behavior either to the present age or to the coming age (which has already started to arrive).
A good way to illustrate this is to think of what happens when we as adults go back and spend several days with the family we grew up in. Many of us find that we revert unconsciously to the way we related to them while we were growing up, perhaps even taking on the “family role” we had then, even though we don’t play it any more in our own household or among our friends. This is not because there’s an active force within us that’s causing us to behave this way again. Rather, there’s an external patterning that takes effect once we get back into that context. The challenge is to recognize that this is happening and choose to behave as the person we have become, even though the social interactions may be influencing us to behave as the person we used to be.
In the same way, the “flesh” as a spiritual force is the patterning of this present age that influences us to act in sinful ways. We need to welcome the influence of the Holy Spirit in our lives to create new patterns that will enable us to live instead as the persons we have become in Christ. This is what Paul means when he talks about putting off your old self with its practices and putting on the new self, “which is being renewed in knowledge in the image of its Creator.” (The language here is not that of sarx vs. Spirit, but of the old self vs. the new self, but it’s expressing the same concept.)
For me, one very encouraging aspect of this new perspective is that it shows there’s no significant part of me that will always resist God. I can surrender my entire being to God in devotion, trusting that all of me will become more and more like Christ through the Spirit’s influence. See how this difference is expressed when we translate the word sarx in a key passage in Galatians using first the old, and then the new, understandings of this term:
The sinful nature desires what is contrary to the Spirit, and the Spirit what is contrary to the sinful nature. They are in conflict with each other, so that you do not do what you want [because part of you is always going to resist God].
The present way of life desires what is contrary to the Spirit, and the Spirit what is contrary to the present way of life. They are in conflict with each other, so that you do not do what you want [because you’re still being influenced by the old patterning].
So I would encourage you to recognize that the Holy Spirit is at work within you to help you develop new patterns of life. The first step, from what you say in your question, seems to be that the Holy Spirit is helping you recognize that there are still some old patterns there that are hurtful to others and not honoring to God. But the Holy Spirit will also give you the power to adopt new patterns and not be held back by the old ones, as you choose on a daily basis to “put off the old and put on the new.” Don’t be discouraged if progress feels slow and intermittent at first; every choice you make will have a cumulative effect and you will see solid and lasting change over time. May God bless you as you seek to “walk by the Spirit and not carry out the desire of the flesh.”
*For the record, in the latest update to the NIV (2011), the translators have changed almost all of the places—nearly 30 of them—where sarx was previously rendered by “sinful nature.” Here’s their explanation for those changes:
“Especially in Paul, sarx can mean either part or all of the human body, or, the human being under the power of sin. In an effort to capture this latter sense of the word, the original NIV often rendered sarx as ‘sinful nature.’ But this expression can mislead readers into thinking the human person is made up of various compartments, one of which is sarx, whereas the biblical writers’ point is that humans can choose to yield themselves to a variety of influences or powers, one of which is the sin-producing sarx.”
You see the difference here between the older view, in which human beings, even after they have trusted Christ, have a component within them that’s known as the “sinful nature,” and the newer view, in which it’s recognized that sarx is instead an external power or influence and that we can choose whether to yield ourselves to it.
The NIV has retained the reading “sinful nature” in only two places, and in each case there’s a footnote saying, “Or flesh.” These are the two places where Paul says “my sarx,” rather than “the sarx,” which could be taken as a reference to something within him, though I think the way he uses the term sarx everywhere else in his writings suggests that he’s talking about something outside of himself in these cases as well.
Good Question 
Is this akin to “Jesus fills our heart when we invite him but grandpa is still in our bones”. Something Pete Scarzo likes to point out in his books about Emotional Heathly Spirituality
Yes, so long as we realize that “grandpa” isn’t actually a distinct part of us, but rather a set of accustomed behavior patterns that continues to influence us. Hope that distinction is clear and helpful.