For my wife and I, my transition has really been our transition--together. You both need to be patient and support each other, which can be hard and unfair to you in ways. Your partner has probably been questioning and running scenarios in their head for years, but you may just now be thrown into it. Thereis also a valid question if you are/will be femme attracted (made easier if you already identify as bi or pan).
But ultimately, if your partner feels comfortable and supported, you'll quickly realize there is nothing to mourn. Your partner is not gone. In fact, they are more than they used to be. My wife likens my past "Tom boy phase" to me still being me, but on mute or low volume. All the things she loved about me shined through more and more as I embraced myself and my transition.
Patience, baby steps, support each other.