Most children of divorce harbor a deep wish that their parents will somehow, someday get back together. They love both of their parents and want them to love each other. And, let’s face it, living in two houses is complicated for kids.
As long as you and your fiance were only living together, this child could maintain the fantasy. With the announcement that you are going to marry, she has to reconcile herself to the fact that her parents are, in fact, divorced and that she will always be divided between two houses. What probably makes it all the more difficult for her is that she is now at an age where she is beginning to understand relationships in a new way. In order for her to feel safe as she explores changes within herself, she wants the adults in her life to stay predictable and stable.
Please don’t take the reaction personally. It most probably doesn’t have much to do with who you are. It’s more about her struggle to understand what it means when people make a marriage and what it means when they decide to part. You might tell her that you know how difficult it can be to understand the changes adults make. Reassure her that her dad’s and your relationship to her won’t change. Be open to her questions and to her feelings. Then include her as much as you can (and as much as she can tolerate) in the wedding plans so she can be part of the transition to a new stage in your family life.
I wish you well.