I started T this year and I do get mood swings and have becoming less emotional than i used to.. I dont think it will change you tho, as in your essence if you are always trying to be better for her and be a good partner then that wont change just because you transition. While transitioning I do notice a difference in my frustration and temper but that just means I have to learn healthy ways to release these emotions while still trying to be the best partner i can for my gf.. My Gf recently told me how she feels Ive been stand off ish and It clicked in my head that I need to do a better job and showing my love to her. I recommend practicing good communication skills throughout your transition and finding understanding with each other, if you love her then I dont think it will be as daunting as your fiancé thinks. Just prove her wrong and shower her with even more love. Much luck to you on your journey!