Hey Xander! First off, welcome to Susan's! It's a great community!
While I haven't been in your exact situation, I'm currently working through issues similar to yours with my girlfriend. I came out to her as Androgyne/Transmasculine months ago, and she had a similar reaction to your girlfriend - the "I feel selfish" and then the not really wanting to talk about it.
What I found works well for me is to be open and honest. I knew it made her uncomfortable, so I told her that while I understood her uncomfortableness I wanted to know what was making her uncomfortable so I could put her mind at ease. Your girlfriend probably is wondering if/when you'll go on T, if you want top or bottom surgery, and ultimately if you're going to be the same person that you were when you started dating.
It took a while to get my girlfriend to really tell me how she felt, but for a while I just talked
at her, rather than
with her. That may be something to try, let her know that she doesn't have to talk about it with you, but you want to at least let her know how you feel. While transition is a personal process, our SOs are along for the ride, so talk to her about your concerns with transition and ease her into it. You may have to do a 10 step process - first get her used to a masculine name, then male pronouns, the idea of you binding, etc.
Let her know that you want her to start calling you by your chosen name and that she's important to you and you really want to involve her in your transition.
Honesty is the best policy with such feelings. Not to mention, in my experience, it's nice to have someone you can work your thoughts out with.