Have you gotten a diagnosis yet? If so, I would explain to her that I have been diagnosed with Gender Identity Disorder. It means that you feel uncomfortable day to day as a male, and that there is nothing any doctor or psychiatrist can do to make you happy other than to change your body to match what your mind feels more comfortable with. If she thinks you're full of it, tell her to look it up, it is a real disorder and nothing that has been tried has made anybody feel more comfortable living as the gender they're born as.
If no diagnosis has been made, idk if I'd tell anybody yet. If you're dying to tell her, maybe tell her you believe yourself to be gender dysphoric and that you are looking for help to try to correct this. This will probably start a conversation that will end up revealing most of everything about it.
Be open but try to be as honest and as technical as possible. People seem to be more understanding if they believe such things are actual medical / mental problems. Afterall, it is more likely someone will be accepting to somebody "with a condition" (Gender Identity Disorder) than it is to accept "a choice" (trans-anything / crossdressing *sounds* like a choice to those that don't know any better, and has many negative social stigmas attached to it, unfortunately).
Good luck and I hope no matter what you tell her, she will be supportive