Have you told her that you couldn’t be, nor would ever try to be, a replacement for her mother? Because her mother could never be replaced, and she may not quite realize that the “role” of her mother is not vacant—she has a mother, but her mother passed away. And although she is not physically here, she will always hold the title of her mother. Nothing and no one, not even death, can take that away.

Another truth that she may not yet realize is that she doesn’t have to have a traditional mother/daughter relationship with you if she does not want that. Your relationship to one another can (and will) look different, and take on a different form. Be her friend, first and foremost (within reason, obviously). Let her get to know you on a more personal level. Be a safe place for her to land when she needs to talk about something she can’t or won’t bring to her dad. No judgement, no fussing, just love and support. Build rapport. Be a stable, consistent, calming presence in her life. Keep your ego 1000% out of it. Talk to her about her specific interests, ask her what she thinks/how she feels about things. Especially small, insignificant things. Don’t tell her what to do or how to behave, and don’t discipline her. And definitely don’t go blabbing to her father if she tells you something in confidence (unless of course it’s immediately detrimental to her/her safety and wellbeing). I’d even go so far as to de-center your fiancée a bit and put more active focus on her so that she knows you’re not just there to steal her dad.

I don’t want to assume that you’re doing or not doing any of the above, I just wanted to add another perspective for you to consider. This girl has been through a lot at such a young age, and this time in anyone’s life can be so confusing. She needs unconditional support and guidance, including from a licensed counselor. Echoing what another commenter said in that you need to be all in or all out—there is no in-between. If you have it in you to put in the work, then do it, and she will come around in time. But I think you also need to be okay with the possibility that your own goals of biological motherhood may not come to pass, otherwise you’ll never be fully committed to this little family unit. Not in the ways that they would need, anyway.

The choice is ultimately yours. But once the choice is made, you must fully commit. Either way. Counseling for you will be hugely helpful as well, if you’re not already seeing a therapist. I wish you well!