Lana,
You've taken the hardest step to self discovery. You are talking about who you are, and what you feel. Dysphoria is all about emotion, intellect doesn't enter into the mix just yet. The suggestion to seek the counsel of a gender therapist is a good one. Finding one and getting into the queue for therapy depends on where you live. The rules vary from country to country. So remember to breathe, know that what you feel and who you are are right and valuable and get ready to take the next step in exploration. My experience is that you are feeling equal parts fear, exhilaration, and worry. If so, that is pretty normal.
You will have to talk to your girlfriend sooner or later, but hopefully after you've gotten some professional guidance. The reality is that for her to be attracted to you, she appreciates the feminine piece of you as a positive thing. There are many paths to an authentic life of which transition is only one. Don't be afraid to explore ways of being across the spectrum of gender identity. You have the advantage of youth, and if you find the life you were meant to live now, it will be a blessing for you and all who love you.
Peace,
Julie