Transgender Support Group
This community is here to support and help the large population of transgender people. If you or anyone you know identify as transgender, this is the place to share your feelings and experiences and speak with others who are going through similar things in their lives.

I am not quite clear and correct me if I am mistaken, did you not start the relationship on erroneous grounds? If you see yourself as a boyfriend and she thinks you are her girlfriend, and you are pretending not to be a boy, how happy can you possibly be?
More the issue, if you are having issues with your gender, do you not concurr that perhaps dating may not have been a wise decision?
Perhaps you could start by going to a therapist well versed in gender issues and get properly diagnosed and go from there. As for your relationship, I do not know how well she will take the news, given she is attracted to girls and you are not a girl, continuing this charade may just make it worse for the both of you. Not mentioning the toll it takes on your psyche, the persistent pretend game you are playing.
You could start to take this time and work on yourself alone with no distractions so that you can have a smoother transition which I can tell you from personal experience will not be easy. Testosterone alone will make your life hell for a while, that is not counting the societal adjustment you will have to endure, the male world is not for the faint of heart.
I understand the need to be loved and cared about yet in this particular journey, we need to set aside a few years, say 5 minimum to work on our issues and learn to love and accept ourselves, so we can have healtjy relationship.
It is however true that even after we transition we may have a hard time finding a healthy mate because we may discover something I have known for a long time, some gendered folks are more messed up than we could possibly be, so we have to weed out a lot of them, no matter, we are perfectly fine being alone than in unhealthy company
If you are wondering why I sound so matter of fact and lack the "there, there care bear" touch, it is to show you what 28 plus years of testosterone does to the brain. Welcome to my world. :)
Hope this helps,
Wolf (ftm since 1987)