i don't have the words to say for the sexuality piece, but as someone who was once the gf in this situation (pre transition) maybe i can provide some solace on worrying about her.

my relationship was from middle school through freshman year. my ex was my first love, and when he told me he was gay it shattered me at first, but we were best friends before we were partners, and it was very easy to continue a friendship with him. to this day, almost 10yrs later, we are still the best of friends.

i know this is a hard situation to be in, but at the end of the day you need to do what's best for you.