Confession pre-supposes salvation. Prior to being justified by grace, I was a slave to sin. I was incapable of not sinning. And, conversly, incapable of perfoming a righteous or holy act.
My responsibility now is to live as the new person I became the moment I was saved and indwelt by the Holy Spirit. That responsibility is to become holy. This is a "this" life process we call sanctification. Even though I am a new person now capable of not sinning by following the leading of the Holy Spirit (God in me). My natural person (Old Adam) continues to be sinful, but now that I am capable of not sinning that is my responsibility. I need now to choose to not sin.
When I fail to be holy, I sin. Confession is the act I perform to acknowledge to God that I understand my sin. In my early faith walk, I am just learning to not sin and am not very good at it. As I mature, I learn to become better at knowing the sin in my life. As I confess my sin, my relationship with the Holy Trinity improves.
When I fail to confess, I intentionally fail in my responsibility to grow in my relationship with God, Christ and the Holy Spirit. As we learn from St. John in the Tenth Chapter of his Gospel, Christ the good shepard will seek out the lost sheep and return it to its fellowship with both Him and the other sheep. The good news for me is that there is no sin so big that it cannot be covered by God's Grace.
When I confess and repent, I am forgiven. As I act to remove sin from my life, I am cleansing it. And, though I fail to act in perfect accordance with God's demand that I be holy, Christ's work on the cross is perfect and complete. When Christ comes again and takes me to be with Him, I will be Glorified and live a completely prefect, cleansed life for eternity.