Re:  why do I need  . . ."

It appears that you are suggesting that a Catholic needs a priest in order to decide if one is in a state of mortal sin.  I don't think that has ever been true, but perhaps I do not understand your point.  I think it has always been up to the individual to examine one's own conscience; certainly, one could and still can seek the guidance of a priest and/or spiritual director to help in the examination.  

Am I missing your point?