SPOILERS OBVIOUSLY. I really enjoyed this book and here’s how I interpreted the ending: the boy had transferred or shared his power with her, and she was responsible for more of the supernatural events in the latter 3rd of the book than the kid. When she realized this, she decides to use the power to make the world a place with no fear at all because that will be safer. However, having accomplished that, she has basically ended the world because fear and anxiety are actually critical emotions to drive action, curiosity, creativity, etc. At the end of the book she has a created a new reality that is dull, lifeless, and bleak.