How we cite our quotes: (Part.Chapter.Paragraph) or (Part.Paragraph)
Quote #10
Periodically, something slipped. Some everyday principle of continuity, the humdrum element that told him where he was in his own story, faded from his use, abandoning him to a waking dream in which there were thoughts, but no sense of who was having them. No responsibility, no memory of the hours before, no idea of what he was about, where he was going, what his plan was. And no curiosity about these matters. (2.258)
Robbie is delirious here, and reality is drifting away from him. In other words, he's losing touch with his story, or his version of reality. This is what's going to happen to Briony, too, we learn at the end. Vascular dementia is going to erase her memories, and leave her adrift.
Quote #11
[…] Briony already sensed that the parallel life, which she could imagine so easily from her visits to Cambridge as a child to see Leon and Cecilia, would soon begin to diverge from her own. This was her student life now, these four years, this enveloping regime, and she had no will, no freedom to leave. (3.16)
Briony's imagining two different versions of her life. In one (which she actually pursued) she became a nurse; in the other she went to Cambridge. Note that, though she says one life disappears, it isn't exactly gone. She's writing about it—it's still in the book. Is the life she didn't lead really less "real" than the life she did leave? Both are just stories in a novel, right? Which is the case for Robbie and Cecilia too. They only live in fiction—but they only died in fiction too.
Quote #12
I worked in three hospitals […] and I merged them in my description to concentrate all my experiences into one place. A convenient distortion, and the least of my offenses against veracity. (4.7)
Briony is saying she's told worse lies. She may be referring to the fact that in her novel she says that Robbie and Cecilia didn't die, but she also might be referring to the way she accused Robbie in the first place.