It might be helpful if you talk with your daughter alone. She might feel more comfortable opening up because it is one-on-one and because you are her mother. She might be more willing to be honest about the specifics of what she is worried about, especially if they are related to your previous marriage or to your current partner. I would start by taking her out to do something she likes, or going into her room, or better yet when you are driving somewhere (children often open up more when in a car or distracted by another activity - throwing a ball around, drawing, whatever she might like to do). It is less intimidating for them. You could start by acknowledging that this is a big change and change is hard for everyone. You can talk about a change that was hard for you or even acknowledge any potential anxiety you have about the change. It normalizes it for her and validates her feelings. Then, encourage her to tell you what she is worried about. If she is not able to say (and is still open to continuing talking), you can suggest some things that you think it might be. I wonder if you are worried about how this will impact our relationship, I wonder if you are worried about getting used to living with a new person, I wonder if you are worried that this marriage will end like it ended with your father? The more open you are, the more likely your daughter will be open. It takes time to open up difficult conversations like this. So, you don't need to push it if she doesn't want to talk about it. Give it time, let her know you are there to talk about it if she wants to and cease opportunities that are calm and try and talk to her again about it. If she is still unwilling to talk about it, you can talk to her about a time when you were anxious about (and didn't have control over) a change. If your daughter begins to open up, acknowledge and validate her feelings, ask her what she thinks would make the transition easier. Once you have some success talking to her on your own, I would slowly reintroduce your partner into the conversation. Use some of the ideas your daughter gives you about easing the transition to be a conversation starter with your partner.
Hopefully this helps answer your question. I wish you all the best and would love to hear back if you have further questions or for an update on how things go. Good luck!