Whether or not you are comfortable with it doesn't matter. Absolutely nothing is going to change your daughter (not son -- daughter) from the way that she is. Do not think for one moment that this is just something that will go away, something that you can ignore. It is your responsibility as a parent to respect and support your child's struggles. Because being transgender is a massive struggle that you cannot even begin to understand.
However much respect and understanding I hold for parents of trans children (because I know, I know how difficult and confusing it is), I'm sorry but your comfort is not a priority. Being that your daughter is still young and dependent on you, YOU have a duty to make sure that she has the support that she needs. It is YOUR duty to ensure that she can express herself with the clothing that she is comfortable in, it is YOUR duty to seek out a gender therapist for her if she so chooses, and so on.
Don't you dare give up on her. Don't you dare ignore her or disregard her or reject her. She is exactly the same as she has always been. You're just referring to her as "she" now. That's literally the only difference.
Did you know that the average life expectancy of trans people is 30 to 32-years-old? Do you know why that is? Because we are either murdered at young ages, or we commit suicide at young ages. Don't give your daughter a reason to want to commit suicide.
If you reject her and refuse to support her, prepare to have either a) a miserably depressed child, or b) a child who hates you and rightfully cuts you out of their life. I don't think you, as a parent, necessarily want either of those things. So suck up your reservations and attempt to learn more about this situation. There are plenty of articles, books, and documentaries out there geared toward helping people understand their trans loved ones. Utilize these things. Educate yourself. For the love of god, accept your daughter and make DAMN SURE that she knows you accept her.