You could say that a person full of grace is a genius...Or you could say a person who is full of grace is extremely homely...Or you could say a person who is full of grace has 3 arms and 4 heads...
The bible doesn't say these things aren't true...
The bible does however tell us what grace is and gives some examples...Extra heads and sinlessness does not fit any biblical description of grace...
But alas...The bible proves you wrong...If the bible didn't address the issue you could have your religion which is mostly based on what God didn't say...Unfortunately for your religion, God says that ALL men/women have sinned...
So I have a choice...I can believe God or believe some Catholic Church father who is calling God a liar...
I chose God...You apparently chose to follow the liar...
Grace helps you resist sin, but God will never force you to not to sin. If you commit sin, however, you lose grace, and are no longer justified. Mary remained never sinned, thus never lost grace and never reneged on her justification.
On the point of all men and women have sinned -- Jesus is True God, True Man. He possesses full Humanity. So did He sin...Or is it possible that you are taking the verse completely out of context?
Read Romans 3 completely.