thebesttofit.
i don't have teens, but i work with them...
you have had loads of good advice here, and lots to think about.
i like the idea of day zero, where a line is drawn and you all start afresh with the new rules and boundaries that you have both agreed on...
it does sound as if your dd may be sexually active, or certainly is interested in having sex, and it would be good if you can give her permission (for want of a better word)to access contraception and condoms...whether she goes with or without you.
i would stand firm on not staying over with the boyfriend, but let her know that he is welcome in your house at any time, with your permission (not sneaking him in at midnight!!)
i think that the secret is clear boundaries, and clear consequences if the boundaries are not adhered to...ie..i'd like you to come home by 10 o'clock tonight please because you need to get up for school tomorrow. if you're not home by 10, then you'll not be allowed on the pc after school/you'll have to stay in on friday night/not be allowed to the cinema with your friends...that way, you give her some control over her fate!!!
i hope that you can work through this...it sounds like the lines of communication are open and are starting to get a little better.