I try to avoid talking about the emotional side of it too much, partly because i always end up distressed and that makes him distressed, partly because I've already said everything and he just needs time to process what I've said.
i do talk about the more practical side - i told him when i bought my binders, i complain when they're uncomfortable, i don't go into a different room to change into them, etc. I don't know if it's making things better but I'm trying to demystify and normalise the situation. I also keep him updated on my GP/ psychiatrist appointments and things. I've been introducing things gradually and asking for advice and opinion, not necessarily approval - more like 'can you still see my breasts' 'does this shirt make me look like a nob' 'can i pull off this hat' 'is packing worth it'.
I am pretty lucky in that he was never attracted to my clothes though i guess, and he was prepared for my chest to disappear because the whole time we were dating I was covering or hiding my chest when naked, or talking about how much i wanted a reduction.
If you approach the subject with the idea that you're not talking about IF this will happen, you're talking about HOW or WHEN, and you're open to advice or valid concerns, you might be able to have productive conversation. Before you talk to her you could work out a (loose) timeline of your transition plans - if there are things that you think are off the table permanently, you should tell her so that she's not worrying about things that will likely never happen. For example (and maybe tmi) i realised that i needed to make it clear to my boyfriend that I'm not interested in certain prosthetics or any of the places i could put them... also it's important for people to know how sudden changes will be, and what they can expect to happen.
things in relationships do change though, and people change - i could have pointed out that as i age and my hormones settle down, my chest would naturally loose a little mass, and since i lost weight it has shrunk. those are changes he might have had to deal with regardless of my gender. Transitioning just seems more terrifying because we tend to think of it as a chunk, with a label, as if it will all happen overnight, when really it's gradual. My dress sense already flipped from tomboy-ish to femme and then back to full boy while we've been dating and he never noticed the first change...