I was a bit surprised to hear this so soon after 479, Like Dolls, by J. Lily Corbie. The two stories seemed very similar in their themes of being trapped and stored for sexual obsession, although the tone of this one was more explicit. Also, both stories ended badly for the captors, but in this case there's much more room for optimism about how the protagonist's character is evolving, as she moves to help the One Below.
I shivered when the protagonist was talking about how it would have felt to be a child wandering and playing in a stream, and then said, but of course that never happened, I have always been here in this room. I felt very strongly then that the box wife DID have some core of original humanity, but was denying her own memories to make her present reality less unbearable.