I've got one too 
Mine is 14 and has turned into the devil child, she used to be so lovely, I don't know where I've gone wrong. Oh, ok, I think I do, I think I might have spoiled her?
She is at boarding school due to logistics. She started just over 2 years ago and loves it. She has lots of friends and I know it was the right thing to do, but the last few months, she has changed.
we took her (and other siblings) on a lovely holiday to Florida in October and she was a stroppy cow a lot of the time and ruined things a few times. At one point she said that she wished her friends mum was her mum as she was closer to her! This is a woman she has met all of about 4 times!
I then didn't see her for 6 weeks until she came out to where I live abroad for Christmas. she was fine the first week but the last two days turned evil again. She has been really rude and abusive and we've just had the mother of all rows just minutes before DH drove her to the airport. I will now not see her until at least Easter and am gutted.
I've done so much for her, maybe too much? I try to do nice little things, I suppose the guilt because I don't see her as often as I'd like. I send her small presents and things to boarding school, magazines etc and before christmas sent her two lovely dresses (which she said she loved and I know she meant it)
I bought her some lovely designer clothes (which she wanted) for christmas and was so upset just before she left to see some hanging in the wardrobe here and was suprised she wasn't going to take them back. They cost me a bloody fortune and I was hurt that she was so blase about them.
Am so very sad we've just had such an awful row and can't kiss and make up.
Bloody kids, they don't warn you what teenagers are like. This will pass, this will pass........