Hi vtor_91,
Welcome to Susan's!
It is really good to see an SO coming here to understand and seek advice.
I am on the other end of a similar situation, as I have recently come out to my wife. We've been married for only a few years but been together for almost a decade now since we started dating. It took us hardly 3-4 months in our relationship to know that we always wanted to be with each other. Over the years, we have planned a whole life together, part of which we have already lived and a big part of which lies ahead.
I am not sure exactly what situation your partner is in, but in my case, I did not even know how serious my gender issues were until last year. I had had a feeling of being 'different' all along, but it was only after a lot of internal struggles that I started accepting myself, and I came out to my wife soon after that.
So far, she has really taken it well, and she says that at the mental / emotional level, nothing has changed, because she has always liked me precisely for those 'differences' that drive my gender issues. But she is not OK with me making any permanent physical changes, or presenting as female to the outside world. She does not identify as lesbian, and even going away from each other is not an option when neither of us has done anything wrong. She thinks we should be able to continue living together the way we always have, while I think we should be able to live together even if I physically transition, but these two views don't really match. So we are in a bit of a bind, and just trying to take it one step at a time. FWIW, she knows about me being here at Susan's and has even shown some interest in what goes on here, so it seems like she may come to accept it more over time.
I don't really have much to offer in terms of direct advice, but I just shared my story in the hope that you might find some parallels. Finally, it is up to your partner to decide about whether to transition or not, and how much if at all. And it is up to you to decide, as others have said, whether you can live with loving the same person on the inside even if she physically transitions on the outside, or whether you want both the inside and the outside to match what you originally fell in love with. Neither of them is right or wrong, it is only about finding what you are comfortable with. And don't worry too much about those labels of lesbian or straight or anything else. They are just labels. Finally, it is your own happiness that matters.