Psycho Female Perspective
First, apologize! Second, apply stokage.You should try to get her excited about how much fun she will have. Tell her that she is going to kill it and she will. Confidence is key for us girls to have a good time. If you are immediately condescending/doubtful and say that she won't be able to keep up or have fun, of course she is going to get defensive.
I'm the same way. I was really nervous for my first pow day and so was MildBill (although he would never tell me that). Instead of telling me that I 'wasn't going to be able to keep up the pace' and that he has 'no friends on pow day', he got me stoked on how much fun it would be and really stroked my ski ego so that I would be super high confidence going into it, regardless of my nerves. I think that the combination of the confidence boost and him sacrificing a little bit by not going super fast, but still keeping a good pace and encouraging me to do the same, really made the first pow day a success. Now we both know that I can keep up and there isn't a 'domestic' everytime a pow day rolls around. She might love it or she might hate it, but at least she'll realize that she either can or cannot keep up with you. If she can keep up, that means tons more fun ahead for you guys. If she realizes she can't keep up, she'll probably let you go enjoy your powder without a fight, knowing that she would be miserable trying to keep up with you.
Review: Say Sorry, Get her stoked/confident, sacrifice a little to accomodate her (not so much that it ruins your day and/or creates an unrealistic idea of what a pow day with you is like), have make-up sex!!!!!! yay!