Well I hope this helps, it's from my own experience years ago. I know the situation has changed with so much easy access to information and impact of social media, but it might give an insight into her head
12 is not very early to think this way. I think for me it started around age 9 when I moved up to middle school which was much more sexist. Girls and boys separated for everything, although many of my best friends had been Male before then. Obsession with boobs and looks in the girls changing rooms. Bullying even from teachers because girls should not be good at stuff, especially maths
So I wanted to be a boy, I was just as good as them. I was always happy to play Zukko if you are of that generation.
As time goes on , In my dreams I was always a boy. Actually, often still am. All the stories I read were sci fi,with no female characters back then either. I just wasn’t into things like make up and the music and tv that other girls liked.
And as for hormones, periods and boobs. Totally horrific. Totally unwanted. Boys get to keep their bodies, I had a razor blade ( used for models) and was very tempted to put it to other use - ideas that I formed without any input from social media
What happened next? My dad let me do lots of stuff that other girls wouldn’t touch , DIY and car maintenance, he brought computers home for me to play with. He always told me that other people were just stupid or scared or jealous if we encountered sexism. He just constantly reinforced the message that I should do what I wanted, and that there would always be friends and family who loved me that way. He turned up at school and forced them to add pure maths to their a level options as I would need it for my future. This school was not the high school associated with my middle school, my parents changed that too for me.
I told this very recently on here, but one memorable day we were servicing the car. And he couldn’t get to the oil filter, but it turned out my smaller girls hands would fit just fine, showing that female biology isn’t always a disadvantage . I was so chuffed.
It took till early 20’s for me to become reconciled to being female. Because no matter what I did, that’s what the world saw. I couldn’t change me to fit in, there was nothing wrong with me , and I was just going to bloody well show them that.
Much older now, and I think I have achieved that. Smug. And unlike those who may chose to transition today, i have my health, which means a lot as you reach the menopausal age. I have achieved without changing me at all, physical or mental.