OP. I wish that everything works for you in your relationship and you are definitely not alone in this.
I will give you some insight about my situation a couple of years ago. It didn't work for me. My ex tried to work with it, but in the end honestly she wanted a man and not a woman. It wasn't really about the sex. Though I still game, I still travel and do most things I did as a man. I am not that anymore and she knew. It is a sexual orientation thing at that point. Was I crushed absolutely. But I am grateful she gave it a chance to see if it would work between us. Please understand this isn't the case for everyone, and I am envious of those that have relationships that stay together.
The only thing is don't force her to accept you, or stay with you, or do idle threats and such. That is just bad news for everyone and most likley will end up bad. However if you are truly happy with who you are and and you need to transition go for it, you just have to be prepared to accept the good and bad though. Whether or not you sacrifice your happiness for hers that is your choice that you have to make and only you alone can make that.
You have to take it nice and slow and try to work it out and if it works out break out the wine glasses and celebrate. If not, at least you know you tried. That parts sounds kind of cold but when it was over between me and my SO, I definitely was crushed, but I also gain some comfort that she tried to make it work and unfortunately she needed a man. She is my best friend, more of a sister now. Not teh best outcome, but I think I was fortunate it ended in a positive way.
This place is always here for help and advice

Jennifer