This one had a pretty big hurdle to overcome for me with the self-cutting. When I was a teenager (wow, half a lifetime ago, weird to think that) I had a close friend who cut herself. She showed me, and told me not to tell anyone, and I didn't know what to do. On top of the visceral, personal reaction, I feel like self-cutting is a misunderstood behavior by most people at the best of times, most people assume it's an attempted suicide which is really a different thing entirely. So I'm very wary of stories that confuse that even further by adding in supernatural elements to an already misunderstood thing.
I think it did well portraying addiction, the urge to do something you know is terribly destructive but every fiber of your being seems to be reaching for it and some details rang particularly true to what I'd heard from that friend, about the pain not being as much as expected and etc. I wouldn't say I like it, but I think it was a reasonable metaphor, and there was plenty of scary and truth about it.
The ending... didn't really seem to match the metaphor that the rest of the story was working along. In the sense of the events of the story it made enough sense, I guess, it's like a parasite that moves on when it's host becomes unsuitable which she does when she contemplates suicide. But in the metaphor of self-cutting, addiction, all that, I lost the thread there. Losing the thread right at the end of a story is frustrating.
In the end, I'm of two minds about it.