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 pinpoint 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.