I'm so sorry you're going through this.

I don't know how to help, really - you can't make someone come to a place of acceptance; they have to get there for themselves (or not).

As others have suggested, therapy or counselling may help her. The other big thing is becoming educated about what being transgender means.

Knowing my partner had to transition helped me - though it was hard at first. It seemed selfish of her to decide to do something that (I thought) would cause our children, myself, her parents and others distress and pain. I couldn't see how we could go from a heterosexual couple to two women and have our relationship survive, it really scared me and broke my heart. That was my initial, knee jerk, emotional reaction. Soon, I realised that it was also very selfish of me to expect her to continue to live in pain and repress herself in order to try to prevent hurting us. I also realised that actually, if she didn't transition, we wouldn't be happy anyway - how could we be if she was hurting so badly. She said at first that she wouldn't take any steps in transition that I wasn't comfortable with, but I knew that wasn't fair, I didn't want to be in that position. What I did was read, read, read everything and anything I could find about being transgender. I found web comics by trans people. I read trans 101 stuff. Articles by trans people and articles by partners. The partner stuff I found at first scared me a lot - they showed very little hope - it seemed that the only relationships that survived were where the partner was already bisexual or actually a lesbian - and even then there was so much angst. I thought we were doomed, but I didn't want us to be. Eventually I realised that the horror stories I was reading we're not like our relationship, they had other problems first, not just the trans thing. I decided to take it one day at at time and had to accept that no matter how much I wanted us to make it through her transition as a couple, that we still might not. I think my point is that educating myself about trans issues, reading so many trans people's stories (mtf, ftm, and non-binary) helped me to see that being trans is just another way of being human. I saw that my partner was not going to change into another person, a stranger, although a lot of what I knew about her and had become comfortable with over the years was going to change. She had tried to tell me that she would still be the same person - but that just made me eye-roll and feel angry. I had to make that realisation for myself. You can't make her understand and accept you - she has to want to try and she has to want to educate herself about what it means to be transgender. Perhaps you could find resources for her to read, but I don't know how she will react to that.

Perhaps show her this sub resist, an let her ask her own questions or read other's stories here.