This is a novel that no-one should review.
It is just too easy to write a spoiler, just a word or a phrase, be it as cryptic as you can make it, that will ruin the journey. Unfortunately I read such a spoiler, and cryptic as it was, I knew where the journey would end. Mostly.
Never mind the implausibility of the medical condition, or the very rare slip of perspective; what Watson has done is to craft a slow journey, through the daily journals of Christine, an amnesiac, and her progress towards understanding who she is and how she got there.
Necessarily, Watson takes some liberties with time, compressing events that in reality might not fit into the time available to the protagonist - some of the visits, and the sheer time for Christine to read the journal each day - for the sake of moving the novel on. Even then, Christine's narrative sometimes drags, as she tends to repeat insights from earlier in the day. It is done, I imagine, to give the reader greater insight into her mental processes, and to build the tension, both of which it does, but it hurts the pace.
If you stick with it - I recommend it as an audiobook to fill a long journey - then you shouldn't be disappointed with the ending.
It is apt.
That is all I will say. There are books I could compare it to, such as ...
But no. As I say, I hate spoilers, and this book is too easily spoilt by comparisons.