How to tell your girlfriend - wow that's a challenge. It sure has been rough for me, since I didn't know exactly what I was when I first started talking about it with my wife, whom I had dated many years earlier as well. I think the main thing that's caused me trouble is exactly that - my wife knew me as a different person. Changing that perception later can be a major challenge. Your girlfriend could be attracted to you for who you *were* not who you *are* now. I'll get back to that later.
I'd get together privately to talk about this. It's not really appropriate in a public place, like a restaurant. Plan for an hour or more just for this, depending on how long your story is that you've chosen to share.
I'm a bit of a wordy person, as you all can see (lol!) so I'd tell the whole story. Make sure you include that it started before puberty (that's if you have at least a few early childhood stories) which will help her to understand it's not just something that came from hormones. Tell her the whole story, top to bottom. At the end, tell her where you'd like to go with it - how you imagine yourself in 5 or 10 years. If telling it makes you impassioned about it, that's a good thing - she needs to see where you heart really is.
Be prepared with answers for things she might bring up like, "I'm not attracted to women", or "I'm not bisexual", or "All I'm hearing is about what you want - what about me? I want a man in my life, not another woman", or "Are you planning on cutting off your privates?"
Getting back to how you have changed since she knew you last: once you see her reaction then you'll be able to understand where the relationship is going. If she 'concedes' and let's you be who you are now, then that's a bad sign she might not allow for additional changes in her mind. Another thing she might do is say, "It's ok, but let's keep it a private thing just between you and me." That's a huge red flag for me, since that pretty much places your decisions in her hands.
You and I both know that's inevitable as you grow into who you are and express yourself more. She needs to whole heartedly accept you for who you are *now*, and in addition, be willing to accept who you might *become* in the future. And, she needs to allow you to do that publicly. You might not want to now, but things might change for you and you want to allow for that now while you still can without as much heart-ache.
I sincerely hope the best for you. Let us know how things turn out.