This is a hauntingly familiar story, MadKarma...My 19 yo daughter was deeply involved with a guy who she basically picked up off the street. She rescued him, cleaned him up (as in bathed), and then before long they were inseparable. She was 16. No efforts on my part, nearly exactly like you describe, did anything to stop her behavior and the deepening relationship. Even after a violent incident which led to an Order of Protection, she still defended him and would also contact him. I could not understand this defense of him, but I now know that heroin was involved and was an integral part of their relationship. I had NO idea then; I was so focused on trying to keep her safe, trying to keep her from running away with him, etc.
All this to say that I have spent many days trying to figure out what I could have done differently, and on good days I believe I did all a mother could do. On bad days, I despair at what a crappy parent I must have been to have not seen I had a heroin addict living in my house...You are doing all you can do right now to help her see a better way, but...because she is 18, she can leave at any point. You can't be afraid of this in her presence. If she doesn't like your rules and you demand she comply, she will turn your life into chaotic misery, this I know. (I let go of all rules at one point as long as she kept coming home. This was my most desperate time.)
Believe me, I understand EXACTLY what motivates you to try to control her life--college, safety, family, safety, fear, and most of all, your expectation that her life should be what you want it to be. These are all completely understandable things for a loving sober parent to want for a child. The hard part is that she's 18, and if there are drugs involved, what would be a fairly benign teenage romance can turn into an obsession, and she really can and may choose to leave. You are used to being the parent who can demand and expect certain things. Totally normal. But soon she will realize you can't do that anymore in the same way because of her legal age. It is purgatory for us, this time between 17-20. And yet, if she won't comply in a reasonable way to the rules of your home, you really might consider letting her go...She will find her way.
A year ago, if I had known of SR, I would have NEVER been able to take this advice, so I understand if you think I am crazy to suggest such a thing. But today, my AD is a recovering addict who is living in a sober house and going to college full time, working an outpatient program, going to NA, etc. She has a new BF she met in rehab. He's 30. It freaks me out. But you know what? She's so much better today than she was a year ago, and she did all of the recovery (minus the ICU hospital stay) on her own volition. We don't communicate much right now, but I have learned that is common for young recovering addicts to pull back a bit from family. Now it is my turn to work my own recovery.
What a journey we are on. I am here for you, and have so much empathy for your situation. Feel free to private message me anytime. Amy