I was really close to both parents growing up, but I was mostly close to my mom. I wasn't feminine in the least really, but when did that have to do with anything? Actually I was usually alone as I got older. I preferred my fantasy... My legos and Legend of Zelda, and my sketchpad and models.
But... Here's the thing.
You had nothing to do with it, and its really not a catastrophe, so don't think of it that way. Its ok to be scared for her (someone who you may have perceived as a son, but has never really felt like one, so its only fair to call them by the pronouns that make them feel more confident and less like a freak. Call them, "her," and, "daughter." That is one of the only ways she'll stop being disrespectful.
I'm in a similar situation with my mom. Its easy to feel resentment when acceptance is hard to get with my own family. I would not have had that resentment if she hadn't said painful things to me, or tried to hide me from the world.
What you need to do is stop worrying WHAT caused this. Its irrelevant right now. What really matters is that your kid needs your support and as confusing and controversial a subject as this is, you must believe them that this is not a phase. Things like this almost never really go away.
The best thing you can do to repair your relationship and understand what she is going through, is to tell them outright that you don't know much about this... To be honest with your kid, and to be as supportive as you possibly can. She has enough to worry about, and in the end the choice to ultimately transition or not will be their choice and no one elses. Do not delay what she needs. Make an appointment with a gender specialist, and make sure your kid knows that you're confused and really scared for her, but that you're trying and that you care.
This isn't a choice. It isn't a mistake, because that would indicate a choice was made. Its not punishment, its not a plague, and whatever it is doesn't matter. What matters is that you love them, and you MUST trust them.
The last thing that I will say is that... When a teenager or son or daughter chooses to tell you something so personal, so taboo, so painful, and a complete contradiction to what they grew up with- They're doing it because they have no other choice. Coming out to you, in her eyes, is the biggest most scary gamble they've ever made. Its an act of faith in you, or because they were so desperate because they can't do this alone. Don't break that faith, and do not make them regret telling you their most painful secret.
Its true. Your fears are logical. The problem is that they probably don't have a choice. The only way to live a life where you don't think the thoughts, "Why am I not a girl? Why was I born like this? This isn't real," All the time 24/7, is to just do it and be what your mind is pushing you to do. In the end, if you deny yourself, you become twice as miserable. If you deny who you are and what you feel like you should be, the pain will be exponentially worse than having a bigot make fun of you.
Your kid is going to have a rough time, but the alternative is a constant never-ending growth-stunted miserable life. They will likely walk this tough road no matter what is done to interfere with their progress. So walk it with them as a mother hand in hand with a quirky daughter, yet loving daughter nonetheless.
If you are curious about what my status is. I am 19 years old and male to female. I've been on hormone replacement therapy for almost a year now. My brain has never felt more comfortable with itself than it is now. Sure I get mood swings, but they're manageable. My doubts about myself usually come from the years and years of living as a male contradicting with my transition, but they never ever last. I am going to art school if I can this fall, and I'm going to go as a girl. My fears about my friends all ditching me, or sending a lynch mob to my house with torches were never really realized. Transsexuality is becoming more mainstream.
Good luck, and be proud of your child. They have risked everything to tell you this, and it means that they still do hold respect for you deep down.
-Kara