Going through transition is a big process. One of the things I heard during the early stages of my transition was "transition is the most selfish thing you can do." I don't know that I agree with this wording exactly, but transition does cause a lot of self-absorption.
I don't think this is a bad thing. Many of us spent a lot of our adult lives paying more attention to everyone else than ourselves. And in order to be able to process all of the varied parts of transition we have to spend a lot of time focused on ourselves.
Changes during transition
A lot changes during transition. Our bodies change, and our personality can change. Our erotic desires might change. And there are big changes in our family, personal and social connections.
Self-focus is only one of a few things that will stress an ongoing relationship. If you are in a "straight" relationship, your gender transition means that your relationship could now be perceived, and experienced, as queer. Conversely, if you were in a relationship with someone who was of the same gender you were before transition, now your relationship could be experienced and perceived as straight.
Of course, you both get to decide how you see your relationship, but unfortunately, relationships don't exist in a vacuum. I am in what we consider to be a queer partnership, but when my partner and I are out in the world, we are largely perceived as a straight couple. And for some people, the shift from one perceived sexuality to another is difficult to handle.
The most important factor in whether or not your relationship can survive transition is self-responsible communication. Both partners need to be willing to own their own feelings, whether they be happiness, grief, confusion, or anger. And, they need to communicate self-responsibly - which means no blame, no shame, no criticism. All of these will increase the likelihood that your relationship can survive this process.
Stay clear about what's most important to both of you. Stay connected during the process, and keep communicating self-responsibly. Own your own feelings and cultivate compassion for the other partner. This is some of the hardest work a couple can do.