I wish there was something I could say that could help others but I honestly think it just became uninteresting to her. It’s just not the
cool new thing anymore.
Like Dr Az Hakeem says, it’s not really gender dysphoria in a lot of these teens, it’s more like a subculture (and an excuse/explanation for something else, eg not wanting to become an adult/having an undiagnosed eating disorder/something traumatic happening either within the family or to the teen themselves (perhaps with a side order of emergent same sex attraction and internalised/societal homophobia).
Unfortunately My DsD changed her name legally as soon as she was able to do so (day after her 16th birthday) and has changed all her ID/opened bank accounts/student finance with that name so unless she is willing to actually admit that her parents were right and she wouldn’t want a Victorian Ghost Boy/Anime character moniker when the phase passed she’s going to be stuck with that for the foreseeable. Still, much easier to live with name regret than surgical regret (the daft thing is her birth name is neutral and is mostly used for males in other parts of the world, so it was always more about reinvention/playing a character than it was about being ‘male’ inside).
re: working with a therapist or counsellor I have come to the belief that it’s better to find someone who is willing to work with the parents as a sort of proxy for the teen.
A therapist can coach you in the best ways to respond to the teens behaviour as well as provide support for you.
it’s a waste of money to pay for private therapy if the person isn’t willing to engage fully and to make improvements, and teens in the thick of this very rarely are.
if there is anything comorbid (eg ADHD/ASD or an eating disorder or perhaps a bereavement) then it may be worth finding support for that but only if the therapist is willing to be extremely neutral on the gender stuff, otherwise the kid will likely just shut down and not engage.
Agree with above that James Esses (and/or Genspect) can probably help you find someone suitable.
Best wishes 