I've been thinking about this topic for a long time, and a friend recently posted a question about this on Facebook, so I thought I'd dive into this topic. I think, really, this topic is about losing one identity, while gaining another. For me, it was going from identifying as a lesbian, to identifying as a pan-sexual man, which is quite a different kind of identity.
Many trans people encounter changes in their sexual attractions and thus identity during and after transition. And with many identities, especially if you identify as lesbian or a gay man, that comes along with it culture and community. The lesbian community, although quite diverse in many respects, with many subcultures, does share a lot of language and culture. And many lesbians, especially in non-urban, coastal settings, have tight-nit groups of lesbian friends (and exes, too.)
On one hand, since I identified and lived as a lesbian for over 30 years before my transition, transitioning meant that there were definitely some things lost. I lost a few friends (thankfully, not many,) and have at times felt an acute loss of community and culture. When I'm in queer spaces where there are lesbians I have to remember, since I easily pass as a straight guy, they are not going to consider me as necessarily friendly.
On the other hand, I've always known that my lesbian identity was not really the right fit. It kinda worked for a long time, but I never felt completely at home in it - I always felt somewhat of an imposter. Feeling much more at home in my own body and life feels so much better to me.
For me, the most important process in healing from the loss has been the trans community. Finding a home among people who understand the journey, and can companion me on it, has been a really important part of my transition process.