I dated a trans man who came out to me six months into our relationship. It was all so strange. This was ten years ago, so although I'd heard of trans people, I had no personal experience.
I knew immediately that if I was going to stay with him I had to be okay with him transitioning. I could love him for who he is without reservations or I could leave him. I stayed and he started on T.
It was fun, intriguing, and fascinating to meet the trans women in his support group, to find out how much hormones can change an adult's body, and to explore things like binders and packers.
As the T started changing his body, I didn't like how hairy his body got. But otherwise I didn't feel like I was missing out. I still got to enjoy an AFAB body. I knew he was male and that didn't take anything away from my sexual interest in him.
My sexual identity took a hit, though. I'd only come out as a lesbian just before I'd met him. It wasn't long before he was passing and his name was changed. I felt like I was being stuffed back into the closet.
I felt guilty for feeling that way because it is important for so many trans people (the ones who are fortunate enough to pass) to be seen simply as a guy/woman rather than as a trans guy/trans woman.
I missed being able to show that I was proud to walk hand-in-hand with my girlfriend. It sounds like a petty thing compared to the misery that is dysphoria and I'd agree it is. His gender identity needed to come first. But it was a quiet little sadness for me.
My family and friends knew that I was a lesbian and that he was trans, but everyone else who hadn't met him pre-transition simply knew him as a guy.
I did feel betrayed that he'd waited so long to tell me. I understood why he'd hesitate to tell me but there was this big part of him that I'd never met because he hadn't let me know it was there.
I felt my options were to either support him or to leave him. Because I was in love with him I wasn't willing to walk away. I decided to give it a try. I would have had more of an honest choice in the matter if he'd come out to me before I'd gotten attached to him. But I was willing to go through something I might not have chosen in order to keep that love.
It worked out for me. I never wished that he'd de-transition. I enjoyed being part of his group of trans friends and exploring his transition. The downside was that the T changed his personality a little. He became more quick-tempered. There were other unrelated problems that eventually ended the relationship.
After we broke up, I found that I was definitely open to another relationship with a trans man. I wasn't eager to go through another transition, though. That much change is hard on a relationship. But I was definitely open to finding a guy who'd been on T for a while.
I didn't regret staying with him when he came out to me. I think of trans guys as good dating prospects. I found that my sexuality stretches to include AFAB guys pretty easily.
On to OP's experience, there's no way that she can honestly say she'll be fine with it because she has no idea of what it is. I never would have imagined half of it when he first came out to me. She's being honest, and that's much healthier for a relationship than telling you what you need to hear and then going back on it later.
If you can help her meet trans people IRL, that's one way for her to start getting used to the idea and get to where she knows how she feels about it.
It's not her job to fix your fears about the relationship ending. She already has a lot to process about you being trans. The best outcome will happen if you don't put additional pressure on her.
If your feelings seem intolerable, that's something that a good therapist can help with. And you may want to wait until the feelings aren't so difficult before you start T because I get the impression that T is a big emotional adjustment.
She's a wonderful person she is because she's willing to get to know you as you really are. It's wonderful that you've brought it to her and you're starting the process of creating the life you need to live.
Good luck. She's still trying and that's all you need from her. Keep reaching out to other support people so she doesn't feel like she isn't allowed to feel her own feelings because she's busy attending to yours. Let her side of the process be about curiosity and discovery.