Steve, its really hard when they go from Daddy's little girl to this teenage alien hellspawn. I suspect there is a lot going on in her life and she's having trouble sorting things out. She's probably striving for independence, testing the limits of your authority, confused by the changes you've made, concerned for the security of her parents marriage and how it will effect her (this is probably a big one), as well as dealing with changes within her body. Some of this is just normal teenage angst, and pushing limits enroute to adulthood.
Individually, these are big issues, and combined with a troubled marriage, can be overwhelming. You say she has no idea of the problems in your marriage, but I doubt that. She's probably picked up on more than you think. She's watching her world crumble and has no way to cope.
I've had issues with my D16. Not quite the same, but similar. I'm at a loss as well. I don't have any answers, but I'm doing what I can to make her feel safe, loved, give her attention, making sure I spend time with her, giving her what guidance I can, and punishing inappropriate behavior when necessary. We also have her seeing her school counselor once a week, which seems to help a lot. She was resistant to that at first, but now seems fairly eager to go.
I suggest just doing what you can to be a good parent. If she's had a measure of independence, I suspect she will resist you exerting greater control, but she will get used to it. Be consistent.
There was a book I read that helped.... "How to talk so your teens will listen, and how to listen so your teen will talk" or something like that. Parts of it were not terribly useful, but there is some good advice in there.
M:23 T:26 Me:53, Wife: 60 S:18 D:16 filed 7/16 W moved out 4/28/17