Not a Slave to Sin
"This passage is one of the most controversial in all of Romans. Since early in the history of the church scholars and laypeople alike have debated just what experience Paul refers to. The debate is an important one, for it influences our understanding and practice of the Christian life." 
So, what's the controversy about? It basically revolves around the "I" that Paul uses in this passage. What kind of person is Paul referring to? Many Christians identify very closely with the picture Paul paints of frustration in this passage, but is he speaking of the Christian life? There are several views:
1. Paul is referring to a normal, mature Christian experience.
2. Paul is referring to his lives under the law as a Jew.
3. Paul is describing the experience of an immature Christian.
Honestly, I could write A LOT on this topic because I think it's incredibly important, but unfortunately this is not the medium to do so. I wholeheartedly take the second view. Thankfully, Doug Moo agrees with me and does a great job of explaining why:
"What ultimately is decisive for me is the fact that Paul’s description of the person in 7:13–25 is contradictory to his description of the Christian in chapters 6 and 8. Note the following contrasts:
“I am unspiritual, sold as a slave to sin” (7:14)
“you have been set free from sin” (6:18, 22; cf. 6:2, 6, 14)
“making me a prisoner of the law of sin” (7:23)
“through Christ Jesus the law of the Spirit of life set me free from the law of sin and death” (8:2)" 
Of course there are issues to deal with when taking this view (like Paul's use of "I am" rather than "I was"), but taking Romans as a whole I think this is the only way we can rightly understand this passage. Particularly, we need to take into account what we talked about yesterday and how Paul has JUST WRITTEN about how we died to the Law and to sin.
So if Paul is describing life under the Law, why do so many Christians feel the same way? Moo summarizes life under the Law like this:
"While delighting in the law of God and seeking earnestly to obey it, Jews were unable to do so. They were held captive like prisoners under the power of sin (7:14, 23). Only Jesus Christ, Paul recognizes, can save Jews from the spiritual death that holds sway over them (7:24b–25a). But until they come to that realization, they will remain captives to the “law of sin” (7:25b)." 
That is most certainly NOT the problem of a Christian who has died to the Law - they are no longer captive to sin. But I think that the reason a lot of Christians still feel this way is that they still act as though they are slaves to the Law and to sin. We're a country of list-makers and achievers. I think a lot of Christians basically think "Grace saves me. I get that. But then I have to be the best Christian possible to make God and others happy." We set our goals and self-improve and kill ourselves doing it. We may the be under "the Law", but most of us are under our own personal law of who we want to be, and our perpetual falling short is what makes us feel what Paul is describing here.
Taking the first view, other than minimizing the work of the Spirit and our newness of life, is just depressing. We have the victory of Jesus on our side and we are no longer condemned. We need not feel this way as mature Christians.
The third view is the most destructive, and a view I think most Christians end up falling into. I leads to this type of thinking:
"Therefore, according to this last view, chapters 7–8 together carry a hidden command: The believer must “get out of Romans 7 and into Romans 8.” Advocates of the first view would argue that the believer is always in both Romans 7 and Romans 8. According to the second view, it is God in Christ who offers to transfer the Jew from Romans 7 into Romans 8."  (emphasis mine)
Let's all agree here: we can never in our own power get from Romans 7 to Romans 8. It's easy to say, but hard to put into practice. We must let GRACE change us. Grace is what freed us from the Law and from sin - it is what will continue to change us. The Christian life isn't about "being the best you can be", but about honoring God in our thoughts and actions out of gratitude and thankfulness, understanding (as we talked about in John 15) that apart from Jesus, we can do nothing. Stop making your own law. Stop frustrating yourself and seek grace. Jesus will change you.
 Ibid. 235-236.
 Ibid. 235