Hi,
I have so far managed to get through many stages of my transition, HRT, FFS, beard removal, hair transplants, coming out to everyone who counts in my life, living openly as a woman and recently filing for a civil identity change with the acceptance (at first) and finally the support of my wife of 8.5 years.
However, I told her about my gender identity issues right at the beginning of our relationship. That, plus the fact that I have transitioned very slowly, first trial with HRT in 2008, upperface FFS and first significant coming out 3 years later..., no doubt helped massively.
However, not only has my transition not broken our relationship, it has actually brought us closer and I think I can confidently say we have as good a relationship as any couple I know, based on mutual trust and support. We also just have fun doing stuff any two women would do together like shopping, going out to concerts, ballets, restaurants etc.. together.
My wife is no doubt atypical and probably couldn't have handled a "regular" guy, (whatever that means

), but I guess it is worth highlighting that people such as her exist. So, no matter what happens in your present couple, transitioning doesn't mean you will forcibly have to spend your life alone.
Wishing you all the best
Donna