They keep saying they wished they weren't but unfortunately they don't feel like a girl or non binary or gender fluid at all. they feel like a boy that everyone sees as a girl.
This line of thinking is very similar to the young male in my family (I can say ''male" as that is acceptable by this relative). This relative says over and over, "I feel like a girl/woman." But how does anyone know what everyone else feels like? Young people, like your daughter, who are struggling w/ self-esteem feel their struggles are unique and different to others. They can't empathize with others and assume everyone else is happy and content.
Your daughter's distress isn't rational, but maybe you can poke some holes in her reasoning - how can any woman possibly know what it feels like to be a boy/man if they aren't in a male body? How do women 'feel' they are a woman? This is such a subjective belief and she's making a lot of assumptions about what women and girls and men and boys feel and what they are. How could she possibly know what goes on in another's head? Pushing this a bit on her might make her realize that she's not as alone as she thinks - that feeling unhappy w/ your body is common among young women.
I have pushed my young relative to give me specific examples of these feelings and I get a variety of very sexist examples, like, "I love to wear skirts." "I have womanly emotions cause I cry a lot." When I counter this person with, "You know DG hasn't worn a dress since 1960 because she hates skirts and dresses, male relative counters back with, "well, that's her choice." Or, "I've never been much of a crier - you know DUncle cries at the drop of a hat," and this person just shrugs....there's a LOT of cognitive dissonance so be prepared for that, but I hope I'm breaking chinks in this person's armor....time will tell.
You DD sounds deeply sad and I'm very sorry for that.