Have you asked your therapist why she believes one of your alters came along at 14? She would be the one to ask about it. It's possible you went through extreme trauma at that age that you are unaware of. It's possible that alter has talked to her about it and you are unaware. Or maybe one of your other alters told her. There's quite a few possibilities,she could even just be assuming or guessing.You really should be talking to her about it,that way you will know.

Authentic self? I'm not even sure what that means. You need to ask her about it.

Sorry I am not being helpful.Most things that are about you and your DID are best figured out with your therapist though.