I wouldn’t be surprised if a whole lot of us have faced your dilemma of wanting happiness for the person you love and willingness to forego your own happiness to make that happen.
I can tell you in my case, it is hard to have a partner be happy if you are unfulfilled. I’m guessing she might have a very hard time staying, especially married, to someone who changes genders in the midst of a relationship. I witnessed one couple where “he” came out to his wife soon after being married. She, the original she, acted as if it was not a big deal, saying their generation has no problem with fluidity. I lost touch with them but I sensed her words were not fully sincere, and I wonder if they are still now a couple.
Now, my story is my wife and I have been married for 43 years. We’ve had our children and they are grown and on their own. I finally decided I needed to live out the rest of my remaining life in the gender I could not be, that’s a separate topic, until now. My major concern was for her also. I didn’t want to destroy her friendships, her life, because of me.
I obviously don’t know how long you two have known each other and how deeply you care about each other, but the fact is you have not yet vowed yourself to each other. In light of the general rate of divorce is near 50% or greater, this strain may prevent marriage from ever happening.
You talk about loving her, and I hope you love yourself, too. To me, love needs to allow the best to happen for the one you you care for. If your loves are strong enough for each other and you can both flourish in a married partnership, then why not go for it, but if there is fear that this bond’s glue cannot hold you together because of your womanly desires, you may need to let each other go. You can still truly love each other and encourage each other as any two women can and stay very close, if not best, friends.
This is only an opinion based on the little you have provided. A solution to your concerns will most likely take some seep soul-searching meditation, and thoughtful discussion with your fiance.
Good luck on your journey.