There was this book that I read when I was young, maybe like 11 years old. A girl went into the mirror in her bedroom and found this world made entirely of glass. There were glass butterflies and stuff too.
I believe she meets a boy, but there is someone or something that doesn't want her there. The world is also dying/in trouble or something and it's turning black. I don't remember much other than I think a bunny dies, and in the end, a bunch of glass crushes the legs of the boy she's with and she has to abandon him to escape.
Another memory of this that's kind of fuzzy and maybe inaccurate is the girl gets stuck in the world for a while but she is able to see into the world through other mirrors/windows and it seems that a fake version of herself has taken her place back in the real world so her parents don't know she is gone.
If you have any ideas please let me know! It was a pretty heavy theme/story for an 11-year-old and I've been wanting to read it again to see if there's any deeper meanings I missed back then.