OK, so the forgotten sin is forgiven. And the forgiveness is not retracted by remembrance.
But the remembering of it imposes a new obligation?
I think of people–baptized Catholics–who have strayed in their youth–maybe a lot, and go back to confession after a span of ten or twenty or thirty years. They make the best examination of conscience possible and then make a general confession of all the sins of their past life.
But confessing “all” is not really possible, is it?. Let’s see, did I fornicate 3 times a week, or 4, or only 2? And how about those times I was just too drunk to recall? How to p(name removed by moderator)oint the number and times of distinct instances of mortal sin which had long since become habitual? How many times, exactly, did I fail to make Sunday Mass? The list could go on and on. And so could the remembrances and flashbacks: “Oh, I just remembered another one; now I need to confess that one. When does it ever end?”
I think it would make someone in such a situation wish to have never been baptized, so that the whole slate could be wiped clean with the pouring of water–once only, with no confession necessary.
If I were a priest, which I am not, I would advise someone in such a situation to make the best confession they could, and then to put the past behind them for good, never bringing it up again, now matter what memories came to mind.