Psychological Thriller Idea

Ever since the release of Black Swan, I have been very interested in psychological thrillers, as I really like the idea of getting into the head of one character and documenting their slow demise. Due to Nina's unfair downfall, I am also intrigued by the idea of a person who has no control over their descent into darkness.

So, here is a rough idea for a psychological thriller:

Shelly, a troubled young woman, has always been crazy, but she has never noticed (or she has been repressing it). However, when her younger sister, Cadie, buys an antique doll from a car boot sale, Shelly is overpowered by her insane urges and becomes convinced that the doll is controlling her sibling. After a series of strange and unexplainable events (mostly triggered by Shelly’s fragile mind), Shelly grows frantic and develops a passionate craving for the destruction of the toy, even if it means losing her family, friends and life along the way.

This idea is very different to Black Swan, but it carries the same concept of a person losing their mind due to means they cannot regulate.