I know what you mean sweetie... I wish I had some magic words to say that would give you the answer you desperately seek but sadly I'm in the same position. Of course everyone has their own variances to it, I guess the only advice I can give is if you know you want to move forward, if you need to, then you have to. It's a scary thought to come out to ones loved ones about this. But remember you didn't ask for this either, just like you know she didn't. And if you can accept this about yourself, then if she truly loves you she will as well.
She may not be able to stay with you after you move further into transition, she may not at all. Sadly all of the research I've done indicates that most relationships end when the one who was born this way decides to right their bodies to match their minds. A lot move on to have a great relationship as best friends, but very few remain together.
I wish I could paint a wonderful picture that would make everything ok, but sadly our lot must go through this process, it's in a way our right of passage, I guess it's the one we must endure in order to replace the one we were rightfully robbed of at birth.
Best of luck to you,
Samantha