While I'm not trans myself, I know people who are, and the fact that you're supportive is so wonderful. Many trans people don't get that from their parents/family and it's extremely painful and, especially if there are others having trouble accepting it, I'm sure your daughter appreciates your support more than you could know.
Still, it's completely understandable that the transition is rough on you too, of course. I would second the suggestion of considering therapy if you think it might help at all. Also, I know in some places there are even support groups, maybe look into those if you'd be interested and can find one near you. If not, there should be some online as well.