Hi rubyhammer,
S10 is likely not sure what she wants if she has a BPD mom who is telling her how to feel, what to want, what to ask for. S10 is a kid, and she has to follow the rules. It's like school -- she might not always want to go, but she has to.
You might also want to read up on validation -- Lesson 5 on this board has a lot of good material about validation with kids, especially validating questions. Add to that the psychological/emotional stuff that kids experience during divorce. Some of the emotional turbulence is part of divorce, some of it is having a BPD parent. It's good to know what's typical so that you can respond in appropriate ways. Validation will allow S10 to process sadness, grief, disappointment, frustration -- this is part of being an emotionally resilient person. "You feel really upset right now, and you're mad at me. It really hurts when we have to do one thing when we want to do another. Do you want to talk about it now, or would you rather talk about it later at home after I pick you up?"
Also, and you probably already know this
but it's really important to stay firm with the court order so that you don't find yourself in court with ex saying "SD10 spends more time with me, so custody should change."
Stay firm.