We need to recognize first the genre of literature of the Psalms.  These are poems.  These are people talking to God, rather than God talking to people.  For this reason, we should not read Psalms to determine Christian doctrine, but in order to understand what a genuine relationship with God looks like.  In this Psalm, David is crying out, in great emotion, to a God from whom he feel separated because of the terrible sin he committed against God, against Bathsheeba and against Uriah.  In this context, David used metaphorical hyperbole to express his remorse and desire to be right with God.  He tells God that “Against you, you only have I sinned.”  This is hyperbole and it is not even true. In fact, David also sinned against Bathsheeba and against Uriah, as Nathan points out.  Then David also says, in a cry of anguish, that he has been “sinful from the moment he was conceived.”  Again, this is hyperbole and it is not literally true, of course.  If we read Ezekiel 18 we are told that we are only held guilty for sins we have committed, and an unborn child cannot sin.  We cannot use an emotional poem from a human to God to prove a doctrine which has been shown untrue from many biblical passages, such as Ezekiel 18.  None of us are sinners from birth.  Original sin, or anything like it, is not biblical.  This passage proves that David is in extreme emotional distress and deeply wants to be reconciled with God. What it does not prove is that David was literally a sinner from the time he was conceived.