For the moment, just let her know that you love her and when she's ready, you want to talk with her about it.
In the meantime, it's worth looking at Dixie's site and http://www.geocities.com/FashionAvenue/1258/ which also has good info for SOs, to help you figure out what some of her concerns are likely to be, and how you might address them.
There's the usual ones: are you gay? do you want to become a woman? etc. as well as whatever issues she may have about what she thinks a man "ought" to be.
Be prepared for her to feel like you've betrayed her trust. It can be crushing to find out your partner kept a huge part of themselves secret from you -- with the implication that they didn't trust you to see it.
I know you've also been going through some rapid changes yourself (with shaving, etc.), which she's probably noticed. So it's likely she feels things are out of control. You may know what your end goal is, but she doesn't -- all she can see is you're actions and no matter what you say, she may be afraid that you're planning to transition out of the blue.
So being honest with her is critical. So is helping reassure her that you're committed to the relationship and will take steps to preserve it -- just don't make promises you know you won't be able to keep (like stopping entirely).
Good luck!