Promising premises, but feels like it starts to fall flat

I was anticipating reading "An Anonymous Girl" since I have read "The Wife Between Us" and enjoyed it thoroughly. I found the plot and the surprise twists in "The Wife Between Us" to be clever, and an overall well-written book, with few flaws. The premise of "An Anonymous Girl" is promising as "The Wife Between Us". There's twists and turns and the characters are cunning, even devious. It also takes the idea of what should be trustworthy individuals in our lives, and how they can manipulate trust and even use it against you, which is a truly frightening concept, in a different way than your typical ideas. It's subtle, and cloying, but equally as terrorizing. But, it starts to fall flat, especially towards the end.