Ashley, I'm 48 also (so you and I and Renate can form a "mid-40's transition club" if you'd like) -- and my story is very similar to yours. I started hormones and therapy as a way of trying to calm the frustration and the anger, and was really hoping that I wouldn't feel better on the hormones because I knew what it would mean. And sure enough, 2 months later, 4 months later, I was feeling great, better than I've ever felt, and I was both sad and overjoyed. Something's coming to and end and something else is starting, and even if your marriage survives, it won't be the same -- there's absolutely no going back to the way you were.
I'm terribly lucky to have a wife who, after 6 months of very chilly and cordial and academic discussion, really turned the corner and is a huge help to me right now.
I don't think it has to be transition or die, as there must be options in between, like androgyny, anti-depressants, and so on, but I tend to think of it in the same way you do -- once you start opening the bottle, there's no way to get the genie back. Going backwards is perhaps the most horrible thing people like us can contemplate, so you're forced to think of what kinds of futures are more palatable than others. There's the one where I'm living as a woman, but divorced. There's the one where we're together, but chilly. There's the one where I don't transition and I'm in a drug-induced stupor, angry and depressed. And so on.
I was reading a book on religion and trans-ness last night in preparation to tell my sister, who is a fundamentalist, and I was literally dumbstruck by a chapter in "Trans-gendered: Theology, Ministry, and Communities of Faith" by Justin Tanis (the chapter 7 is entitled "Gender as a Calling"). "Rather than seeing ->-bleeped-<- as a medical problem to be corrected, a psychological incongrence between body and spirit, or even a quirk of societal organization, I look at my experiences of gender as the following of an invitation of God to participate in a new, whole, and healthy way of living in the world--a holy invitation to set out on a journey of transformation of body, mind, and spirit." I'm not religious at all, but I was struck by how you can change the terms you use to look at your life and that simple change makes all the difference. I remember thinking something similar about 4 months ago: "what if being transgendered is not a curse, as I've felt all my life? What if it's a blessing instead, and by "giving in" to it, I'm not losing a battle, but actually winning?" It was a weird flip-flop of perception, but it has helped a lot.
This may not help you at all, Ashley, but lots and lots of us have been right where you are, and choosing life, choosing to listen to your inner calling, is nothing to be ashamed of. You can PM me if you'd like to talk. Your wife, even though she's private, might benefit from joining a discussion board for SO's with a pseudonym so she can vent, too.