Sorry to hear of your difficulties; hope a workable diagnosis arrives soon to help all of you.
We have just had an exceptionally difficult Christmas Day with our AS son, 7. Yes, he is 'high-functioning', but it is very evident today that he needs a completely different Christmas than the one we have been giving him so far. Our DD (5) is fine.
We are going to completely tweak next year so that it is truly happy for both him and us, and for every year that it needs to be different. We can't wait.
I hope when the Christmas pressures have died down that you and DH are able to formulate a New Year plan to get a diagnosis and/or tackle this as a team. It's the only way, for your DD's sake. So interesting to see that both of you have been worried, but I infer, hiding your worries in different ways.
Come clean with each other about your worries, have a good cry, get cross with the world, have a hug, and then get straight on with helping your DD.
OK, she is not what you had imagined, but most importantly she is your DD - and once you have been through the stages of grief (denial, anger, grief and finally acceptance) then accepting and adapting to her differences will become second nature - although this takes time.
I still look at my AS son, in the top group of numeracy at school but having a complete anxiety-stricken meltdown as he did today, Christmas Day, which was very severe, and have to pinch myself and say - 'No, this is how it is - he is NOT coping and is VERY different - we need to do things differently'. Three of us in the family are 'normal' (though I think on reflection I am AS myself, but can cope!) but we need to accept that one of us (DS) severely isn't.
It isn't the end of life as we know it; it does hurt; but it is all possible and a happy life can go on for us all - happier once we know the problem and can take on board strategies for improving it.