It has been sad to read through the last 4-5 pages of this thread seeing the words of those who seek justification based on their obedience to the law, but it completely makes sense that they would downplay their own sinfulness. They have to.
A Christian living in light of the gospel can be honest about his sin because he knows that his righteousness if found in Christ, not himself. His weakness does not bother him. It's expected. But one living by and under the law has nowhere to go with his sin, he has to turn a blind eye to it because it condemns him, so he does one of two things, and usually vacillates between the two. One, he becomes proud when he sees himself living inline with God's law, and two, he becomes despondent and defeated when he doesn't.
The main trick he uses, and he has a few, is that he keeps his eyes on others rather than on the actual law of God. His measuring stick is others, not the law. This is why he is judgmental and condemning. He has to be in order to bolster himself up.
The second trick of one living under the law is that he lowers the bar of the law to make it achievable. He fools himself into thinking that he is either without sin or that he sins only very occasionally because he has taken the glory out of the law. He has lowered the bar and fools himself into thinking that he is walking in compliance with the law. Along the same lines, he makes other laws about things such as drinking alcohol or watching certain movies. . . This helps him in his self-deception to live (in his mind) in obedience to the law.
Lastly, a common theme with the few people posting in this thread who are imagining themselves with very little sin is that they only see sin as wrongdoing; however, they fail to also see it as a lack of right-doing. They fail to see that not meeting their neighbor's needs with all of the immediacy, concern and resources that they meet their own is the same as lying or theft.
The reason I know so much about these types of people is that I used to be one of them. Praise Jesus that He forgave me the arrogance and insult to Him and His grace. Thank Christ that He rescued me from such a life. He saved me from trying to justify and sanctify myself. What joy, peace, and gratitude have replaced fear, condemnation and judgment of others. . .