One of the reasons I've proceeded as quickly as possible from starting HRT to completing GCS is that my male hardware doesn't work as well on estrogen. My experience isn't exactly in line with yours, however it's completely in line in some main points.
While I'd certainly say I've enjoyed sex for most of my life, it took more time than most people for me to get started and like you I always put my pleasure in terms of pleasing my partner. As I approached the decision to transition, sex became less about penetration and orgasm became more elusive, and so even before I started to consider HRT I'd arrived at a place where I rarely orgasmed during sex with my partner and I'd most often get her off and enjoy the closeness that came from sex play, and I'd finish myself after she'd had her orgasms.
If I'm marginally less attracted to her over time it's mostly because I know she sees me as masculine and still enjoys that I am muscular and have a <shenis>. And since HRT, of course the equipment doesn't work as well (Viagra can help with the mechanics but I fundamentally need to be in the right emotional space to become aroused now).
I may also simply need to be paired with a lesbian over the longer haul, our relationship has in fact improved as she's accepting me as transitioning, sexually, she may never adapt to a more top role though I'd like that to be an outcome of our shifting relationship. Fortunately our relationship has been open from the start so there's no guilt to going to others.
I've know I was bisexual for much longer, however I'm really not attracted to masculinity in any emotional way and I'd be very surprised indeed if I were ever to have a satisfying ltr with a male. Sexual play is another matter however it's an itch I only scratch when circumstances bring it to me, not something I go looking for.
Keep us posted with how things go. I think some changes in the sexual calculus of a relationship are inevitable when we transition, however that's true of lots of forms of changing relationship.
