When I say "I have sinned", I mean that I have been party to a physical event where I hold doership as an actor.
Sinfulness is not this act itself, but the feeling that flows on inside us, of regret or guilt, and colours our present day image of ourselves with a dark brush.
We cannot undo the original sin, if we choose to call it that. What we can do is to remove the tinge of sinfulness from our memory of the sin, so that we are whole and pristine again, free to deal with life with our fullest energy.
Sometimes an act helps to drop this feeling of sinfulness. It may be a confession, or a reparation, or even a ceremonial bath. The memory then no longer haunts us but just remains as a reminder of a path not to be taken again.