My best friend had this happen and she let her daughter stay with her ex but told her she had to stay a minimum of 12 months (no backward and forward) to avoid playing one off against the other. Her daughter was back after those 12 months as Dad was not what she thought he was. He was living with his parents and couldn't be bothered even spending time with her while she was living with him. He left the parenting up to his mum.

Anyway her daughter came back and is a better person for having been through this. She is currently in Year 11 and has a part time job and is doing really well after having some serious problems earlier (with the law).

Don't let her walk all over you and set the ground rules now. If she goes to her dad's then it is for a certain time frame and she cannot come back until after that time expires. No matter how much you want her to.

Good luck making this hard decision

Kim