This poem can be interpreted many ways. Is she just kidding herself and she is the girl in the mirror? Is that who she is, inside, and she's just bottling it up? Is that someone entirely different than herself and she literally just walked away? Is that someone she once was and she was strong enough to walk away from the tough times? Is that someone she isn't and she had enough self-righteousness not to assume that that is who she is?

That is up to you.