Why does she feel she needs to leave? it's not as if she can live with boyfriend, if HE says no. And if she chooses to have sex with him, clearly it can't be under your roof or his. So why should it matter to her where she sleeps?
I think you did amazingly when you didn't engage her, when you said, "You're an adult, you are now responsible for your own choices." That was amazing, and just what the situation needed.
We've been down this road with both our girls. We had to accept that we had done the best we could in handing on to them our morals, our views, our code of ethics - our kids get to a point where we no longer have influence over them, they listen more to their peers and follow their own desires. All we can hope is that we instilled enough moral fibre while we had the chance. And if we didn't instil enough - it's too late now. The best we can do is keep the door open so we can always talk, she can always come ask for advice knowing she won't be judged. Especially with easy child 2/difficult child 2 at the moment, we have to bite our tongues so much... but we figure, if she's not receptive, why say it? Instead, we say what we think she WILl listen to, and stay silent otherwise.
On the subject of taking practical responsibility for the decision to be sexually active - I took my kids shopping for contraceptives, I took them to the doctor and got them into the routine of regular pap smears, breast checks etc. There's something about being in a supermarket with your mother and hearing her ask, "Do you want ribbed or unribbed? If you get flavoured, avoid the banana ones, they taste horrible..." to totally take away the "we're young and we invented sex just for ourselves" myth.
easy child 2/difficult child 2 used to tell the local kids that she was a witch and would threaten the more persistent boys that she would put a curse on them if they continued to bother her. Mostly though, she let other kids continue the rumour. I had Christian friends coming to me in a panic over this, but I knew it was all a sham. I had talked to her about witchcraft, explained the LACK of magic behind it all (you can generally find the psychology) and showed her how to find the 'trick' in books she read. There's always a catch, which can be used to 'explain' why something doesn't work. I also showed her the arrogance of thinking you have anything about you which makes you more special or more superior to other people - the real damage lies in the attitude of "I'm special because I can do magic" when the reality is, "I'm an idiot because I've fooled myself into believing utter nonsense, and I'm wasting my time on this when I could be out there living a fulfilling life."
She still dresses like a Goth, but has totally discarded any interest in the supernatural, apart from pure academic interest in gullibility.
She's no longer drinking to excess (which she did when she got the chance, for a couple of years) and won't do drugs.
But the big thing - she simply isn't ready to leave home. If I said to her, "You can't sleep with your boyfriend under our roof," which was our ORIGINAL position, until we realised we couldn't stand guard forever, we had to sleep sometime, and if they were going to do it perhaps it was better done in the safety of our home than in some backstreet gutter somewhere), then I know she would leave. And make a total mess of things AND refuse to talk to me about any of it.
The Christian in me would ideally like to be able to say, "My kids have made some good decisions with their lives, they have chosen to be celibate until marriage and they are good, honest kids working towards a fulfilling and productive life," but I know this is just a pipe dream. So we have adapted to what we have - and the result is, for us, kids who know they can talk to us, kids who still have problems but who we work to see through them, kids who are honest with us. And the other really good thing we hadn't counted on - we get to influence the BFs too. The girls bring them home, we get to meet them. BF2 has moved in with us, he has changed a great deal for the positive in the last year.
I look at friends at church and their families - it's not all roses for them, either. Some parents seem to cope with it by denial ("What problem?") to the extent that they don't even know themselves sometimes (but their kids talk to our kids, and WE know). Others are like us - their kids have made their own decisions and the parents are accepting that this is how it is.
I also have an advantage - I'm the youngest of 8, by a big margin. By the time I had kids I was seeing all the mistakes being made by my sisters. I was almost the same generation as their kids and they talked to me, so I heard it from all sides. I've since used that information to try to hold my own family together.
It's not going to be perfect. The best you can do is try to make sure you can be there for your daughter, so she can come to you and talk especially if/when things go pear-shaped. And if you can't make it that good - at least you have tried. I think it's the best we can say.
One of my best friends was a teen mother like you. She has been in a big panic over her daughter and her daughter's choice in boyfriend (he's an abusive creep). The mother was terrified her daughter would make the same mistakes, and as a result she tried to be too controlling. Her daughter is a lovely person, although I agree with the mother - the girl has made a bad choice in boyfriend. But in her panic to see her daughter safe, my friend is risking driving her daughter towards the very person she wants her away from. I've tried to tell her - welcome the boyfriend as a guest in the house, give him adult status (hey, he's ten years older anyway) and be polite to him regardless. Because in doing so, this boor's bad behaviour to them, his bad attitude, will have absolutely no excuse in the daughter's eyes.
Currently, the boyfriend can say to the girl, "Your parents hate me," and the girl believes it because her mother has said, "We don't like him, we think you could do better."
But if they welcome him, are kind to him, cook his favourite meal when he comes to dinner and he STILL says, "Your parents are mean to me," she can look at the situation and see the boyfriend more clearly as a liar and manipulator.
And I think somewhere in my friend is still the teenage girl who felt totally shamed when she fell pregnant while unmarried. She constantly feels unworthy and has poor self-esteem. I keep telling her how gifted I think she is, I remind her how many languages she can speak, but she still sees herself as belonging to a lower caste. I think that could also be part of the problem - "We have to work harder to lift ourselves out of the mire, we have so much to make up for."
Sorry to ramble - not sure of any of that could be relevant to you. I hope you can use some of it. Maybe when we get to know the fine detail of your story more... it takes time. I'm glad you're here though. This is a good place to land. And your input for others will also be welcome.
Marg