A girl sits alone in a dark room, watched only by her own reflection. It is quiet, but the tension in the air is as thick as old custard. Will she pull from the depths of despair or will her shadow overcome her?

I started writing this story a while ago at a crime fiction workshop, and decided to continue it. I would like it to be a full-length story and I need some help. It's about a girl battling depression and anxiety while being watched by her reflection (the narrater),  but I don't know whether the reflection should have a part to play or if she's just watching. All I know is that she is evil and likes watching her real life counterpart suffer. Any help?