1 John 1:9...Psalm 51......
The Pharisee stood and was praying this to himself: ‘God, I thank You that I am not like other people: swindlers, unjust, adulterers, or even like this tax collector. 12‘I fast twice a week; I pay tithes of all that I get.’ 13“But the tax collector, standing some distance away, was even unwilling to lift up his eyes to heaven, but was beating his breast, saying, ‘God, be merciful to me, the sinner!’ 14“I tell you, this man went to his house justified rather than the other; for everyone who exalts himself will be humbled, but he who humbles himself will be exalted.”(Luke 18:11-14 NASB)
I am not perfect and I do sin on occasion.
If you think it's "on occasion" and not daily then you might need to rethink things.
I go to a priest and confess my sin, if I am truly sorry.
Are you saying there are some sins for which you are not truly sorry? If so, how are those viewed by God? Are you forgiven if you confess even if you're not truly sorry?
He will judge if I am sincere or not.
What if you're a good actor? Some of the most sinful people can play the part of a contrite person....understand I'm not saying you do.
If I am, he will absolve my soul from that sin and give me a penance. Doing the penance shows God that I truly am sorry and it is then erased from my soul. I receive a grace from God to know I am forgiven and that I truly want to live as He asks of me.
What "work" could you do that would equal, or even come close, to accomplishing what the sacrifice Christ made on the cross did for us? Jesus is the reparation for our sins. He is the one-time sacrifice. He has paid our sin debt nailing it to the cross.
I would like to see your biblical support for "doing penance".