Mewoman
When I read your post it brought back memories of a similar problem with my niece (26).
My niece seems to be in full recovery now - 1 year sober and doing better. BUT - it was after two felony convictions for selling crystal meth and all the drama that goes with that (jail, workhouse, court appearances, etc.)
She spent about 2 years in what I call "the lifestyle" and the past year recovering. What I learned is that I had to start with myself getting help and then I knew better what to do for her. Which was letting go, and letting God and consequences teach her.
Some things I can offer to think about:
If she has money for drugs, she has money for rent.
You have the right to require that no drugs come into your home. You can set consequences (but make sure you stick to them if you do).
If they do random drug tests at the facility, it is only a matter of time before they catch her. Weed stays in a person's system way longer than other drugs.
The boyfriends are just another symptom of addiction and a life gone astray. You also have the right to keep the boyfriends out of your home. My niece, when she was using, had a whole string of strange folks in and out of our home. Now that she's back, we don't let her have anyone overnight (even girlfriends) and I won't even meet anyone she dates until she's dated them for 3 months. (none have lasted that long). This keeps the drama level down at your home and if you don't allow the boyfriends over, she'll either slow down or move out.
It is imperative you stay calm if you take this route. Be prepared, however, she may decide not to come home many nights - so you'll have to weigh whether you'll sleep better with strange men in your house or her being gone and not knowing where.
While she was using and selling, I did a lot of worrying, praying, rescuing, fixing, etc., until I finally was able to let go. I had to stand by helplessly while she was beat up at gunpoint, arrested, had an attempt made on her life (the folks who did this are in prison now) and other horrible things. But in the end, it was only getting caught and sent to rehab and jail for a while that woke her up.
If I had to do it over again, I would hope that I would let go and trust God more. My daily prayers worked - I don't know of much that I did that made a difference.
The only thing that helped was when I set limits and absolutely stuck to them. I kept telling my niece that when she wanted to get sober, I'd be there for her, but until then, she was not welcome to live in my home. I would see her from time to time, give her a meal and a hug, but beyond that I held my boundaries.
For me, I just knew that if she didn't stop the drugs she'd be dead anyway. Selling is dangerous so if the drugs didn't kill her the lifestyle would. And the longer they use, the more irrational their thinking becomes.
My suggestion for you is to find a nar-anon family group and start going. It helped me a ton. There's nothing like talking to others who have been through it to keep perspective, give you hope and help you set the limits necessary for yourself and her.
I know how heartbreaking it is to watch someone you love systematically destroy their lives. My niece just could not see the error of her ways until she was sober for about 6 months. Until then, she was angry and rebellious.
I pray that your journey with your niece is not nearly as difficult and that you can find the support you need to help you make the many difficult decisions that lay ahead of you.