Opinions on my new story idea?

It's an interesting idea, but I suggest you do a ton of research on Multiple Personality Disorder. Mental disorders are very tricky to write properly and insufficient research can cause a whole mess of problems. Example: the name of the disorder you're talking about is Dissociative identity disorder (DID for short). It's name was officially changed to better reflect the disorder. It's also treatable and treatment is generally very successful, especially in younger patients. So I suggest you start with some very thorough research on the mental disorder, then see if it still fits your story.