Okay but “what’s fair” to a 9 year old child takes precedence over what’s fair to a parent, I’m sorry. Yes it’s not your bio kid, but when you decided to become a parent, and did 9 years of parenting, you opted in to protecting and taking care of this helpless kid. Yes she’s not yours, but you are her dad in her mind and there’s no undoing that now. Being a parent is hard, being cheated on and lied to is hard, having what you thought your life was shattered is hard, shared parenting and maybe getting divorced is hard - but none of that stands up to being a child who’s only dad she knows and loves abandoning her. That is trauma that will affect her for her entire life. So no it’s not fair, but OP you’re an adult and her dad and like it or not you do have a responsibility for how this turns out for her.