OP, I totally understand how you feel. Even since my kids have turned teenagers, I felt in the middle between my OH and my children. Unfortunately, the concept of a happy family was not going to happen in our place, which is fine, it doesn't always work, but it means that I am constantly in the middle trying to satisfy the demands and needs of one and the other and sometime, it's overwhelming.
Rightly or wrongly, you've given your DD a lot of attention and she became dependent on it, and now she sees it dwindling which is of course upsetting and scary. At the same time, your relationship is still new and of course you want to give it attention.
My advice would be not to try to act like a happy family all three together. Your DD and your partner probably prefer to spend time just with you, so do priviledge that and value quality over quantity.
I don't agree at all with 'she 17 and it's her time now'. Kids don't see it this way and shouldn't have to. If you hadn't met someone, you probably would still be happy to have the same close relationship with her. Also at 17, they still need attention and guidance from parents. If anything, I would say my DD needed me more at that age than when she was 14/15.
Listen to your DD and don't make her feel that now that you found someone else to make you happy, you don't need her as much. Your OH needs to understand that you are mum before anything else and he needs to respect this. It won't be for much longer that he'll be able to have you almost all for himself.