You are so right, just because I have to act a certain way around my girlfriend does not mean that I have to be like that all of the time. I should use this time to get to know myself and my friends better and let them get to know who I really am. I need to get over looking at the big picture all of the time and take the small strides with a big heart.
For me it was making up my mind to take the lumps as part of the learning process. The biggest part of that was getting out in public. Starting out with masectomy breast forms and a wig was tough - I couldn't even spell passing let alone do it! To force myself out the door I made it a practice that anything "Diana" wanted she had to get; whether it was laser treatments, clothes, or even a cup of coffee. Ninty percent of becoming just another woman is the confidence that you are.
I can't begin to suggest how to act around your girlfriend, but to be honest with both her and yourself, you need to talk. Plan on the worst, and hope to be surprised. Finding the right words, and coming out to family and loved ones is perhaps the most female-affirming thing you can do. This is where transition really begins - not with clothes, hair or hormones. This is where the woman begins to assert herself in order to survive.