For all you know, what you want your daughter to "get" may have already sunk in. I remember knowing by the time I was 14 what my limits were, and they were solid, even though I squirmed for my mother's embarrassment every time she tried to bring up the subject.
But since you can't know for sure, it won't hurt you and may help her if you gently persist. Don't make a "big talk" out of it, but remind her whenever the moment seems right that you are there to answer questions and support her. And express your need and responsibility, as her mother, to let her know what you hope for her. Bear in mind that if what you say is different from what you do (i.e., if you are sleeping with your boyfriend), she may be less likely to take you as a credible source of guidance.
Sex is intensely personal (as in "secretive") to teens, expecially if they are engaged in it, or tempted to become engaged. Unless you have made this an easy subject to bring up from the time she was little, she's pretty unlikely to want to talk about it now.
Every teen is different, but I'll bet most of 'em would prefer to get their information from a "neutral" source (non-judgemental, unemotional, and anonymous), and for many kids, that would be a good book or two from the library. You could even purchase one or two good ones and gift them to her, perhaps acknowledging your confidence in her desire to be responsible for her own well-being. Google "books, teens and sex" to get some good suggestions.
It sounds like you are on the right track, in spite of your anxiety. I do have a suggestion about your approach. Instead of saying what sex SHOULD be, which could provoke skepticism in your daughter if her experience or longings are different (thus making it even harder for her to talk about it), I'd be inclined to describe an ideal of what COULD be. If your own experience was different from that ideal, you could perhaps talk about your regrets, and how you would choose to do it differently if you had a second chance.
Good luck, mama. I remember well the white knuckles and knotted stomach that would sometimes come with being the mother of a teen girl. But most kids turn out all right! Mine sure did!