I recall this being a short story, possibly in a Gardner Dozois collection that I read in the mid-2000s. The protagonist is a young woman, living at home with her parents. She overdoses on a 'zombie'(?) or 'z'(?) drug that's used to give your unconscious mind control over your conscious body. In doing so she effectively kills herself, despite her body being up and walking around with an entirely different personality in control. The main character constantly talks about her former self in the third person, as if she's inhabiting someone else's body.
Her parents are concerned and threaten to send her to some kind of rehab facility. At the end she agrees to go, or at least considers it.
