Harvestdream, it's not always an easy thing when creating an extended/blended family unit. One thing that I realized when I remarried was that my children, no matter how much they liked my new husband, had some very extreme/mixed emotions about our marriage. Children will always dream of their parents getting back together again. They can't help it. When someone else comes along in one of their parents' lives, they begin to realize that mom and dad are really done and not getting back together. They feel crushed, confused, angry, sad, depressed etc. etc.
I think your future step-daughter may be acting cooler towards you because she is not sure how she feels at any given time. She's still trying to figure all this out. I think, being as patient as you can be, acting normally around her, active listening to her will help her feel more comfortable and may even give her the opportunity to speak openly with you when she's having difficulties.
Children want their parents to be happy. Children also think that when the parents divorce, it's because they did something wrong. They will do what they can to try to "fix" it. It's so important for the children to know that it never was their fault that their parents split up. Maybe you can have that conversation with the child and their dad all together?
Just be prepared for good and bad to happen through this. Their emotions will go up and down as will yours.

I wish you all the best and hope that you can find that gentle, caring place for each of you.