Your daughter is lucky to have a parent like you who cares so much and is listening to her. I understand this is very scary and completely unexpected for most people to see their child going through. How could you ever foresee such a rare and poorly understood situation? We don't learn about gender and sex as two separate things. We don't learn about lgbt as natural, normal variations of human sexuality and gender identity, at least not until recently. No one mentions gender dysphoria in childhood development literature, even psychology textbooks get half their information wrong. And there is a lot of fear and hate out there for people with any type of less common difference from the kind of people who don't like to color outside the lines. Your daughter may have some extra hurdles to face in life. This is the bad news.
The good news is that this is one of the most exciting, rapidly advancing times in American/Western transgender history: we have great hormone treatments and very good surgical treatments (if needed, she can avoid some surgeries like facial feminization and hair removal by delaying or stopping your daughter's male puberty with hormone blockers), people are starting to understand transgender in a way that they never have before, and the medical community is waking up to the call for accessible, adequate treatment. It's very reasonable to expect that your daughter's lifetime will see a dramatic improvement in the civil liberties of trans people and that many parts of the Western world will be a safer place for girls like her. For girls her age, being trans isn't a tragedy, not like society used to make it.
And with a supportive, informed parent advocating for her, your daughter is more likely to grow up to be a happy, healthy adult. If you dive into this now and get a firm understanding of what your daughter needs, she can avoid so many years of suffering and loss from going through the wrong puberty and then facing transition in an unsupportive, hostile environment.
No one would wish these challenges on her and your family, but if there was a better time to be born trans in American history, it had to have been before any European colonists settled here.
I would connect with your local trans community to find other parents and community leaders to support you as you learn about what is best for your family. You're not alone in this journey and there is a lot of hope!