This may be considered heresy, but for my :twocents: , the first celebration of this sacrament is as much a pastoral event for the penitent as it is a “legal” sacrament. I believe it is important for a penitent to be allowed to confess whatever may be troubling his conscience – however long ago in childhood, however trivial. I would say that if you are questioning your own scrupulosity, then you may already be in “safe” territory.
The fact that we are not required to confess venial sin does not detract from the fact that our most troublesome burdens may be at the level of venial sin, and as Augustine says, the weight of many grains of sand can ultimately bring you down . . .
I heard somebody say once that a good general (life) confession should not take longer than six minutes. That may be technically true, but pastorally it could be brutal to put an egg-timer on this event. That said, some historical models might not be worthy of imitation. I think of Ignatius of Loyola, who took 3 hours, and John Henry Newman who took 7 hours over two days!
This is not a contest. Just commit yourself to perfect honesty, perfect integrity, perfect authenticity: do not embroider; do not make much of nothing; do not make nothing of much. Let the Holy Spirit take the lead. A good priest with experience in this will help you through it. BTW, although I was counseled that “childhood does not belong here,” I found that, in fact, when I revisited the sins of my childhood, I saw that they were far from trivial or inconsequential, but that they had seeded the sin-history of my adolescence and adulthood. FWIW.
I can promise you: your first confession should be one of the best experiences of your entire life.