Good point about the forgotten sins. I definitely understand your concern. Scrupulosity can be such torture to endure, especially if worrying over being damned for forgotten sins.
I see the quote you’re referring to, but the book isn’t unequivocally stating that we’re in a hopeless situation of being damned for sins we’ve forgotten (and I totally see your point that this is potentially confusing/a mixed message).
The book recalls the story of Saint Vincent who was visited by a soul from purgatory who was bound to make reparation after death for his slanderous words about Saint Vincent. We don’t know whether that man recalled what he’d said about Saint Vincent or whether it was a forgotten conversation, but he was nevertheless bound to make reparation, and in God’s divine mercy, was saved and not damned.
I know some overly scrupulous Catholics who, despite their problems with scrupulously, will gossip, speak ill of others and tear down reputations through detraction. This can be a grave yet often overlooked sin even among the scrupulous.
It would be a shame for a scrupulous person to avoid this book out of fear of becoming stressed or upset by its contents, when the alternative would be for them to remain unaware of the gravity of the sin of detraction and, if they’re guilty of the habit of the sin of detraction, to continue in this mortally sinful behavior.
I’m just happy to find a resource that brings to light the moral obligation that–as difficult and humiliating as it might be, and how much our nature rebels against it–we are bound to make reparation if we have hurt another’s reputation (and to do so during our lifetime on earth if we recall it, and if it’s within our power to do so).