If she is gender fluid and you supported her masculine expression, then her leaving has little or nothing to do with your gender—which means you are off the hook as far as any transition guilt. It also means the two of you weren't meant to be together.
Sure it hurts—I've been there, and raised two children from my first, busted marriage to adulthood. You will survive, you will thrive, you will forgive yourself and her. You will meet and get to know the most important person in your life—YOU! That's right, the girl in the mirror.
Hormones are going to take you on a wild ride, totally blow your mind, and really introduce you to yourself.
Only after really knowing yourself will you really be ready for an intimate relationship. And the time will come and good things will happen.
Cherish this time of self discovery. Afterward, you will realize it was over way too soon.