For I know my transgressions,
and my sin is always before me.
I think that most of us have experienced this before. Think back to the last time that there was some major sin in your life. Didn't it seem that that was always at the forefront of your mind. It was always there, it was always right before you.
But David didn't only have it present in mind, he had a daily reminder of his sin. Bathsheba, the woman that he had committed adultery with, the woman whose husband he had had murdered, was now his wife. She was there to daily remind him of his sin.
So he cries out for mercy. He knows his transgressions. He knows that he can no longer live this way. Nothing can hold him back. He must be right with God.