When we do mess up, we need to confess our sin
to God that just means that we repent (which means agreeing
with God that what we did was wrong) and ask forgiveness.
1 John 1.9 says, If we confess our sin,
He is faithful and just and will forgive us our sins and purify
us from all unrighteousness.
It is true that God has already forgiven us,
but by confessing specific sins after we have received Christ, we
restore the relationship that has been broken by our sin. Because
we are already forgiven, we dont need to worry about making
up for it when we sin...all God wants is the restored relationship.
Our sin may still have earthly consequences -- a murderer still
has to do time in prison, for instance -- but before God you have
a clean slate!
Confidence: How to Know if We're Saved