It is very hard what you are doing and I can only imagine how difficult it must be for you. I can tell you that my daughter was 15 when I was throwing her a birthday party shortly after her father and I separated and I caught her in bed with one of the boys that was at the party! I was furious and in a bad situation because I knew that she was partly doing this because she was angry at her father for leaving, but was also confused about sex in general. I called the boys father and made the boy leave and the following day I had a talk with my daughter. Years later she told me that she doesn't remember one thing that I said except that she was going to give herself a reputation of a slut if she continued to do the things she was doing.
Girls are not easy (I can say this because I have one of each) but with lots of patience and prayers you will get through it. She's very angry right now and probably feels that you are only taking care of her because you have to - find a way to make her understand that you love her and what to give her the kind of live that her parents wanted her to have. Let her know that you understand how she feels and that it's ok for her to be angry. Just be there for her.