Fiorella has traced that guilt she told you about--deep inside herself, she still believes that, when she was a very young child, she killed her grandmother.
She's told you about it before, how, as her grandmother lay dying, Fiorella was told to stay in the large foyer outside the front room. She remembers staring at the umbrella stand, at the front door, the stairs, and around at the empty room, and she knew her grandmother was in bed in the room off the other side of the front room.
Fio can only guess that, because she was alone, she probably entertained herself by singing.
Someone came through the door--a woman whom Fio, of course, cannot identify--and told her she should be quiet because her grandmother was dying. So when Grandma passed on soon afterwards, Fiorella assumed she had killed her by making noise in the foyer.
Yes, she actually believed she was a murderer, and she kept her horrible secret for forty years--until she finally faced the memory and realized there was no way a child could kill a person by making noise.
Unfortunately, though, the belief is still embedded in Fio's psyche.
But strangely enough, it has helped Fio become a better person than she might have been otherwise. She knew she had to tread the straight and narrow because she had already committed such a terrible crime, a crime she had escaped rightful punishment for. It also gave an overload of responsibility because she apparently had great power. Yes, Fiorella is out to save the world.
A final thought. After looking at the dates, Fiorella told you it was her grandmother who was dying, which occurred when Fiorella was four and a half. But through the years, she always thought it was her great-grandmother who was dying, which occurred when Fio was two and a half.
Take your choice.