I'm not sure if I am reapeating any advice from anyone but I will just say what springs to mind. My friend went through this with her dd who is ASD and also most likely lesbian or bi.
She was very shocked at first (the mum) and tried total denial, which did not work.
She looked into binders and decided no, but allowed her dd to get some sports bras, they went together to shops but some girls may choose to shop alone.
Eventually the mum became very supportive of the fact her dd is almost certainly gay (apparently lesbian is a word young people do not use much these days, very sad I feel, but better to try and use words kids understand).
My friend was very loving and reassuring that she loved her dd and would whatever happened (after a very difficult initial period of denial).
It is sometimes possible to talk without using the child's name or female pronouns (if this causes distress) and say things about them as a fabulous child or person rather than daughter or girl.
If you Google around you can also read up on ROGD (Rapid Onset Gender Dysphoria) which I am sure someone has mentioned and there is even a forum for it somewhere.
There is a parenting LGBT children section on Mumsnet, I guess I would take advice with a pinch of salt. Welcome advice from all quarters for yourself but do not show your dd anything you don't want her to see.
This is something that passes for some.
But I would suggest you don't say that or push it because being told this is just a phase is going to make her (most likely) hang on more tightly.
Maybe watch this in a quite moment alone and I hope it makes you feel better...
Penny White talking about her daughter who wanted to be a boy at one time
PS She is still your lovely child, give her lots of love and reassurance, and go slow, slow slow. Haste is, I think, your enemy.
Finding fun and love and happiness in the everyday and being patient will help. And if one day she really does transition in some form, by then it may not feel so terrible and also whatever happens, she really will still always be your child. XXXXX