What do I do about my out-of-control 12-year-old?

I need supportive, informative, helpful answers here. I'm desperate. My 12-year-old daughter is beautiful and smart and I love her very much. However, she is completely out of control behaviorally. I don't know what to do anymore. She had sex for the first time recently with a 17-year-old boy she just met that same day (he's in legal trouble now, but that's a different topic), she smoked pot with some high school kids when she was supposed to be at a friend's house playing, she steals from me, speaks very disrespectfully, gets detentions at school all the time, and is failing most of her classes (she's in the 7th grade). I'M WAVING THE WHITE FLAG! I'm in so much pain and have been crying for the last two days straight. Let me say: I know I'm not a perfect parent. I'm human and I make mistakes. However, my daughter lives in a loving, safe home. She has everything she needs and much of what she wants. I tell my children I love them every single day (my other child is very good and gets good grades). Nothing traumatic or dysfunctional has ever happened in our home. I'm at a loss as to why she's like this. As for steps I'm taking: I did get a phone number for a counselor in our area that provides intensive weekly therapy, and I'm calling tomorrow first thing to set up counseling for her. Also, next year for the 8th grade, I am pulling her out of public school and sending her to a religious school down the street, in hopes they will have a better group of kids instead of the negative influences she seems to be around at her current school. I have tried heart-to-heart talks with her, I've taken all of her privileges away one-by-one (now she's not even allowed to hang out with friends or use the computer). Doesn't make a difference in her behavior or attitude. I used to think only neglectful or "bad" parents turned out kids with these issues. Please, please, PLEASE someone....give me good, useful advice. I need it.