Hello Rebecca: I'm sorry your daughter is having such a difficult time.

I read your post yesterday, but did not reply because I didn't think there was really much of anything I could offer. I still don't think so.

But I noticed no one has replied to your post. So I decided I would.
Unfortunately the only thing I can really think of to say, I'm sure won't come as any news to you.

At 18 years of age, your daughter is now basically an adult. Perhaps the time has come to set some ground rules she must abide by if she is to continue to live in your home. (I presume she does from reading your post.) And if she prefers not to abide by your ground rules, then the alternative is for her to find somewhere else to live. (If in fact she doesn't live with you, then perhaps it's simply time for you to let this go... difficult as that may be.)
I'm sure this sounds harsh.

But the fact is your daughter is only going to get older year-by-year. And from what you wrote, it sounds as though nothing is likely to change with her.

That being the case the likelihood is you're going to end up having to "lay down the law", so to speak, eventually anyway. So then the question becomes do you do it now or do you postpone it... perhaps to the point where you do in fact have that nervous breakdown you feel you may be about to have even now? Only you know the answer to that question.
The cold hard reality here is no matter how much you love your daughter & how much concern you have for her safety & her "success" in life, you can't make it happen for her. She is the only person who can do that for herself. No good will accrue it you allow yourself to dragged down in her undertow. I wish you well...