It's a nice thing you're so very receptive to her needs, you know. PTSD leaves us with slightly less than zero ego so it always, always comes as a bit of a shock when someone DOES show they're actually listening, and responding to something we've said.
I think Amethist did have a good point, in that we'r awfully given to making sweeping statements we mean at the time-perhaps mean for awhile but it doesn't always have to be carved in stone. Perhaps knowing the pressure is OFF for any expectaions about Christmas, she'll at least possibly not have an awful day-possibly a relaxed one, who knows? Since you do not live together, do maybe get your own tree up, quietly, if need be, and some decorations. Yes, you're together but if Christmas means something to you personally, at least do take the time to mark it's arrival for yourself if you can. It can be done respectfully, with no fanfare but well enough so you do not miss you own holiday. Plus, since she has a daughter perhaps it will be a nice treat to see things festive at your place, and the proverbial spirit of things there since her mother is so fragile at the moment.
Maybe her perception and memories of what Christmas is have been cancelled- obviously something there is terribly painful. Hopefully another type of day for both of you can sort of be invented, free of expectation, stress, memories she can't bear and whatever else caused her to make that statement. Some peaceful sort of Saturday , steak and salad instead of turkey, her favorite music instead of carols, you know?
It's tough to tell what will 'work', I know, but as a sufferer I would like to tell you that the thing which made the single most difference for me was bumping into the man who did just hung around and persisted in loving me. Healed so much- can't describe the difference this made in my life. Huge- as in HUGE. Everyone's different so don't wish to give you sort of 'false hope', but can only tell you what this did for me, personally.
All the best to you, and her. Do take care of yourself also, while taking such good care of her.
Anni