Teenage daughter going off the rails since I left her dad

My husband and I decided to separate six months ago and we have one daughter, whos 15.

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At first things went smoothly - we were civil to each other and our daughter seemed to accept things quite well.

We did all the stuff you're meant to do in this situation - long conversations, reassuring her we'd still be around for her as much as ever and it would mostly be about getting used to the new living arrangements.

We decided she'd stay with me during the week and see her dad every weekend, plus we planned some things to do together as a family.

I thought wed cracked it, but recently things have been hell. She's been acting out a lot at home and at school, arguing with us, shutting us out and going AWOL with her friends.