Horrible. As a parent of a “transgender” kid, I can tell you that it’s definitely a mental illness.
I think in the case of my son, he was always a bit “different” and somehow latched on to this idea. I think, unfortunately, that he got on the internet and did some “research” and found out that he “must be” transgender because he didn’t fit in. We took him to counseling...and he was on ADD meds, too. Not sure how much of that aided in his “transition”.
I’m so very sorry for your difficulties. There isn’t much worse in life that problems like that with one’s child. We have a ton of problems with our son but not in the same category, so I understand the anguish.
Some very dear friends of ours have a beautiful daughter who first decided she was lesbian. Her “marriage” fell apart and her “mate” departed. She then decided she was trans and is now embarking on that journey. Life sure does throw you totally unexpected curveballs.