I did not, for many reasons, follow my transgender drive, and the longer it is left the harder it becomes.
Once on HRT you change, if you choose to go down that path, after anything from a few months to a year, physically and from a sexual perspective irreversibly.
I agree with wild flower in principle, but in the real world its not as simple as that. If your GF is very heterosexual, then she may drift, unless you and here get into a looser relationship.
I can't give you any real advice, except, if you were her how would you react?
It takes a very strong relationship to withstand full transition, however, having said that following girls in this forum I am comforted that the stay together rate is higher than I imagined.
L Katy