‘Sin’ is the nature to sin, but ‘sins’ are not the nature to sin. If we confess our ‘sins’ He is faithful and just to forgive us our ‘sins’, and to cleanse us from all unrighteousness. If we confess that we are sinners God doesn’t re-forgive us for that in some undefined way.
God forgave us for being sinners one time when we were convicted by God the Holy Spirit once and for all resulting in our salvation. Confessing our sins one to another is not confessing the sins we have committed to each other, but it is confessing the fact that we are sinners to each other or that we have sinful natures. If we confess our sins (not that we are sinners) we are forgiven because God says He forgives us and not because we ask His to forgive us.
Confessing to God that I am a sinner is something I will not be expected by Him to ever do again. He wants me to confess that to you though, and I do. Confessing sin acts I have committed (or not having done something I knew I was supposed to do and it therefore became a sin act) is something I do every day in my mind with out making a big production of it.