In the midst of Paul’s internal struggle with sin, Paul talks about his internal problems of sin. In particular, Paul talks about how when doing sin, there is “sin that dwells in me” (7:17). In fact, Paul even talked about how regarding his flesh, “nothing good dwells” (7:18). Considering the condition as revealed in Romans 3, our flesh couldn’t meet God’s standards of good, but Paul could also be considering even a low bar of goodness. Long story short, Paul knew that he is messed up.
As an example of how Paul knows sin’s dwelling, Paul talks about how he fails to do the good he wills and vice versa (7:19). Paul’s saying in this verse (as well as a few verses earlier) reveals how various sin can be instinctive to the point that it can get ingrained in our subconscious.