Dropping the T-Bomb is something best done face to face. I was on my drive home from my third ever TG support group meeting still feeling totally floored. Still feeling that I needed to be there. And knowing it is almost too late to tell my wife what was up. You may want to prepare a few notes. Having tried that myself I can guarantee the conversation will quickly divert from the script.
One important point to keep in mind is that you have likely spent an entire lifetime and barely have a handle on what your feelings are today. As you have said, what you felt a few months back is even different from your feelings today. I can practically guarantee your feelings, your visions of the future will continue to change as time goes on and you continue to grow and learn who and what you really are. Your spouse has had mere milli-seconds to process the T-Bomb and is seeing her entire image of herself, her past, her future, the shared future explode.
All the filters may be shut off as she viscerally responds. The reptilian brain in complete control. You will need to be strong and try to filter her message from the words. Listen for what is trying to tell you or ask you, not to the words. Assuming there are words and not a total meltdown.
BTW - "I don't Know" is a perfectly valid answer. Saying "Today I feel that_____ but I cannot say if later on I still will". She will be looking surety. Something solid to latch onto after being totally blown away.
What has saved, so far, my marriage is the open honest and often difficult discussions. A big aspect of my transitioning has been learning how to talk. To try to express feelings in some sort of coherent way to her when I could barely describe them to me.
And, of course, prepare for the absolute worse case response. These sort of talks often don't go well. Even with a spouse who knew she was married to a guy with gender issues settling on being a CD as it was in my case. Anger, betrayal, feeling like an idiot for being "fooled". "If only I knew...." "All these wasted years...."