That's a question I have. I mean if everything is allready forgiven on the cross, why do I need to ask for forgiveness when I have sinned?
One thing that can be helpful (I see some helpful things already in some early posts I read), is to read the parable of the Prodigal in full, and notice more which one hasn't yet, or hasn't thought of lately. He was originally alive, a son of his father.... right? Notice though the conclusion and the description of him being "dead" and then restored and "alive" "again"!
We need this whole chapter really, because look:
3 Then Jesus told them this parable:
4 “Suppose one of you has a hundred sheep and loses one of them. Doesn’t he leave the ninety-nine in the open country and go after the lost sheep until he finds it?
5 And when he finds it, he joyfully puts it on his shoulders
6 and goes home. Then he calls his friends and neighbors together and says, ‘Rejoice with me; I have found my lost sheep.’
7 I tell you that in the same way there will be more rejoicing in heaven over one sinner who repents than over ninety-nine righteous persons who do not need to repent.
Already a sheep to begin with, and then lost, and then repenting, and thus restored.
Luke 15 NIV
One way to know when one needs to confess is to pay attention to that feeling of having done wrong, and not to ignore it or put it aside, but then to confess.
To show fruits of repentance means one will be noticeably changed also, which has to be real change inside. Already when we confess, we get the amazing grace of being cleansed, and restored. But we still have a need to avoid temptations, pray for help (the Lord's prayer), and walk in the Spirit, instead of the flesh.