Hey, I have the same with my Mother! She also says there were no signs (there were) and that I will regret this, and that a Mother’s instinct is stronger than anyone else’s opinion and she knows I’m just not trans (I am!). She also sent me some “literature” (transphobic articles / videos) in the past.
I felt similar to you after being faced with such a strong rejection of self. She would say things like I was destroying my deadname’s personality, jeopardizing my soul, and that I had changed. All stuff that kind of churned me up and introduced insecurities where I didn’t expect them. It’s normal to respond to this kind of rejection this way, and I also had very similar thoughts and worries about how maybe she’s right that I’m not really trans and I’m faking it. But when I sat down and really thought about it, I was much happier trans (with euphoric moments similar to what you said), and that I finally felt free. Fortunately friends were willing to listen to me as well, and rightly point out that my Mom is saying horrible things to me.
I too regretted ever telling her with how she reacted, and how upset it has made her. But, realistically, whenever you came out she would have responded this way. This was something you’d have to go through at some point. Forcing yourself to hide who you are only becomes a harder burden, and wanting to share who you are with your parents isn’t a bad thing. As a friend said to me “Just coming out is a great coming out”. You shouldn’t feel bad for telling them who you are, nor should you feel bad about who you are because their reaction.
Unfortunately if your experience is anything like mine, these kinds of messages may continue. I’m a scientist by trade, so I would meticulously tear down her arguments and attempt to show where her understanding was wrong, but you can’t reason someone out of an emotional position. It will just cost you time and energy. “The only winning move is not to play” and put up boundaries. Putting up boundaries is something I’m currently doing, though I expect I will have to go low/no contact with her given no change. That being said, I do know of other cases where parents have changed and accepted their child, and maybe that will be the case with my Mother and your parents.
I hope things will be work out for you though, and honestly hearing that your experience is so similar to mine will help me deal with any insecurities that may be stirred up as I deal with her. We’re both trans, and we should both be proud of this, and no amount of parent rejection can take that away from us.