One of my pastors explained to me as a young believer that we aren't even aware of all our sins..it's like we are in a totally dark room with thick blinds and God allows a thin sliver of light to fall on the floor, exposing some of our sins like dirt on the carpet..we clean that up, and then God expands the light in the room slowly, showing us more sin. We clean that up, and it goes on and on like that.
You are right, though..if anyone thinks that they will be sinless by the end of their lives, they are crazy. I am reminded of grumpy older people who don't show the respect and patience that they should be showing toward younger people. I've done it myself sometimes. And, there are issues in our lives that God hasn't shown us, mercifully. I believe if he threw the blinds wide open, we'd collapse into a heap and probably wouldn't be able to continue on, if we were exposed to his full holiness and how far we fall short of it.
God is merciful that way
That's why I find sinless perfectionists to be hilarious. They set up 3 inch hurdles for themselves and think they are some kind of star athletes because they can jump over those. They obviously don't understand God's holiness or they wouldn't be making such boastful claims. Jesse Morrell and those Pelagian/Moral Government Theology guys are like that. They are some of the most boastful, condemning people I've seen (I won't call them Christian because I don't think many of them are true believers yet).