I remember it from years ago. Pretty sure it was a book, not a short story but could have misremembered that.
It was told from the point of view of a girl in hospital. She was narrating her memories and daily routine. They were keeping mirrors away from her. A big reveal about a third of the way through was that they put a mirror above her bed and revealed that she was a chimp- who had the girl's brain transplanted.
Can anyone remember this? Google is not helping