Christians will sometimes talk about 3 uses of the Law. The first use is that of a curb, where the simple threat of punishment (either Divine or human, temporal or eternal) will keep people from behaving badly. The great theologian Governor Tarkin understood this - "Fear will keep the systems in line, fear of this battle station." This first use is quite useful - it's sort of the basis of much of our legal system - and even the most wicked person can be brought to heel by it (although, as the above mentioned Tarkin doctrine shows, it has limitations and can be prone to abuse).
The second use is that of a mirror, a theological use where the Law shows man his own sin, showing man that he is not righteous as God created him to be and desires him to be. This is the what is sometimes called the theological use of the Law. As a theologian, this is vitally important - if one does not know his own sin, one will despise his Savior. If you insist you aren't drowning, you aren't going to care about the life preserver.
Then there is the 3rd use of the Law. This is often called a guide or a trellis. The idea is that for the Christian, the one who has been brought to faith, who is redeemed, who has the New Man in him, can look at the Law and learn from it, can shape his life according to, and view the Law as a guide for structuring his life.
This is true.
However, until the resurrection of the dead upon the last day, as long as I am in my flesh, I am not simply a new man, I am also a sinner with the old Adam. The problem comes in when we teach the 3rd use of the law not thinking or understanding that any use of the 3rd use, of what I ought to do, will automatically also be 2nd use of the Law showing what I have not done.
We forget this. "Here is how you go and live your life as a good little Christian" is rightly heard as "I haven't done that - ergo I am a bad Christian".
For the person with the burdened conscience, the Law ALWAYS has this 2nd use overtone. It always hits like a hammer.
And while you have flesh and blood, it should always hit you like a hammer as well. If it isn't. . . why not? Why do you think so highly of yourself that this correction of the Law is merely a small thing compared to the wonders of getting the better rules for living? What do you think of yourself - and is that in line with what the Scriptures say?