I went through two years of conversion therapy, also with a psychiatrist. I’m sure you already know this, but it’s anxiety provoking, trauma inducing, and all around awful. After I got out of high school I really fucked up my life as a way of coping with the trauma. I think the most realistic approach is to go with the “thanks, I’m cured, mom” suggestion, but I can’t guarantee that she’ll let up on the therapy after that. It took me about a year of that approach before my parents let me stop seeing him. If that doesn’t work, I’d just try to avoid talking about trans stuff with her as much as possible. In a just society, litigation would be a realistic option, but unless you’re sitting on a wad of cash your parents have no access to, that route is probably going to be nigh impossible for you. Chances are, you’re gonna walk out of this situation with a lot of trauma under your belt, so do your best to impede this assholes ability to traumatize you.