Molly, as Stephanie said, you never can tell until it happens.
My wife knew about my cross dressing before we were married and was generally unsupportive: "I don't want to know/see/hear about it" was her attitude. When I admitted to her that I might want to go further than cross dressing about 8 months ago she became very angry and reacted in a way that seemed very selfish to me. She took a, "What's this going to do to me, my life, etc." position. You have to know, in order to understand this, that my wife was born with cerebral palsy and while she's very capable, always expected me to take care of her the way a "man" should in case her disability increased. The next few weeks were as difficult for her as they were for me. She was upset and depressed about my revelation. I was upset and depressed because of my self-revelation three weeks before. I had convinced myself that I was a well enough adjusted cross dresser (I gave up feeling guilty about it many years before) and nothing more and when I realized there was more to it I basically lost my self identity. I didn't know who or what I was anymore. It was a very uncomfortable situation that lasted quite a few weeks.
Over the months, however, we've looked at some TV shows about ->-bleeped-<- and I purchased and we both read Mildred Brown's book,
True Selves (which I recommend) and when calmer heads prevailed we were able to discuss the situation more thoroughly. At first she was so upset that she couldn't even look at the book but as time went on she read more & more of it.
Now, she is a little more used to the idea and a little more supportive. She is concerned about the resources that this process will use up and she has said that if I decided to fully transition that she will leave although I think (hope) that position might modify itself, if past experience is a guide. She does allow me to express myself more fully at home now, which is a real big step, but she's still uncomfortable with my going out en femme. We live in a small city and she's very concerned about being recognized or read and earning other people's poor opinions. She grew up here and knows a lot of people. I'm a little further on than that, though, I could care a lot less. "I yam whatteye yam," as Popeye said!
Now, I'm mildly hopeful that the relationship will survive. She accompanies me when I go to therapy because, she says, she doesn't want me to drive all that way home while possibly being upset (It's a 90 minute plus drive, one way). This gives us the opportunity to do some sessions together in the future. I've taken the advice that I've read here at Susan's and in other places to be patient with her (and myself!) and to reassure her more than usual, that I love her and need her and will be here for her and that I will not ever leave her. I told her that if there's any leaving to be done, it'll be done by her.
I'm sure that you are very frightened right now, if my past feelings are any guide, but I think you're going about it in a much better fashion that I did. Remember, and don't forget to tell her a lot, that you love her. Be gentle with her and patient. You've been dealing with this for a long time, no doubt, but it'll be brand spankin' new to her.
I'll be looking for your post about how well it went.
helen
PS - Don't forget to BREATHE!