This is quite dark, but I wouldn’t really worry about marking it mature…
Humdrum, whether it be a bearable one, or the perpetual torture this story illustrates, seems to be a good grounding for these kind of stories, but this does take it a new way :)
All I can say with regards to challenge suitability is that the story of how she gets herself into this circumstance is begging to be told, but, still…
I like the story, it wasn’t particularly a startling revelation at the end but who needs that, sometimes its what happens in the body of the tale that matters.
If you made the sentences shorter, and uspet a few literary geniuses by throwing in a fragment or two here and there, I think you’d be able to really up the tension and make the pace a lot more frantic.
This is the kind of tale where you could have us breathless by the end of it.
I agree with SJ and MH’s comments. Also, I find that a title can sometimes hurt a story, by explaining too much of what someone is about to read. This seems to be the case for this story.