I lost my partner, we were together for 7-8 months after I started my transition, she was supportive but in the end she told me she wasn't attracted to me any longer.
All breakups hurt but I feel like they hurt even more when both people still love each other, the reality is however that some people like girls, some like boys and some like both, you can't change that and it plays a major part in our selection of significant other.
I know you are hurting but it gets better, remember that logically knowing this could happen and processing it emotionally are different things.
- Hugs