I know this is so hard. I have a stepdaughter who has made terrible choices and it's so painful.
Here's the thing though. If you make it clear that you don't like the guy, you drive her to him. So back off on that. If you don't, it will make it impossible for her to come home to you if this all goes south. You MUST be a soft place to fall, a welcoming place if she changes her mind.
So you do the hardest thing: you put a smile on your face, you act like you have raised her to be sensible and sure of herself, and you admit to her that she is 18 and can do what she wants. You tell her that you support her. You tell her that you've thought over your position and you think you have been too judgmental and (dare I say?) controlling (knowing full well that your issue with this guy is that he is controlling. You say nothing about his education - there are other people who have not graduated high school. If he has a good job, that's what matters now. You understand that baby birds need to leave the nest even if their flying is not so smooth.
Next you offer things you don't care if you don't get back - a frying pan, some leftover tupperware, a few towels, whatever.
Then you get your name off every bill that she has, and your put in a forwarding order at the post office for her mail, all of it. Every car and medical insurance bill, everything. You give the new address info to AAA, the cell phone carrier, the school if she is still in high school, everything. No exceptions. She's 18, she's an adult, she wants to handle it all. Great.
You have your house rules and those stay, so she doesn't come to stay overnight or mooch off the meal schedule, or anything else. You can meet her for lunch someplace (split the bill) or meet at the movies - anything to keep the lines of communication open. You stay off the subject of her boyfriend but if she brings something up, you don't criticize. You say stuff like "Tell me more" or "How is that going for you?".
If things fall apart because he is abusive, you just ask her how it makes her feel when he says that stuff. You don't judge, you don't say, "Just leave. What's wrong with you?" And you don't say "I told you so." No matter how many times you did. You just keep telling her that she is a smart woman and she will make decisions that are best for her.
If she can't afford her life, you don't give her money. You say what every other person does in that situation: "So sorry you are having difficulties." If he is so controlling and doesn't let her do stuff with her friends (or her family, for that matter), you say, "Gee, you used to be so close with Natalie and Georgette. Do you miss them?" Your remind her that she is smart and capable and that she will find a solution. You say she's an adult now, as she has convinced you, and you are staying out of it.
If it's really desperate and she wants to come home, you pause. You say, "Well, that didn't really work out for you in the past. How would this be different?" And you don't make a decision. If she's in danger, she goes to a shelter. That will be the hardest thing you will ever do. But she will get help and objective guidance from trained professionals. You cannot have a dangerous man in your house - which is the first place he will look for her. You must keep her safe, and that means going into a concealed location. You haven't said that he's violent but he shows so many other signs of being an abuser, I lay it out there for consideration.
You have to let go. This is really hard but it's really important. She has to see her life with this man for what it is. She said she is going for a few days but she took everything with her so you both have to call it what it is - she has moved out, and moved on. Her room is not hers anymore. I'd go ahead and clean it out or repaint it or make it into a sewing room or a guest room or a craft area. Anything so you are not looking at what once was. It also lets her know, if she should ever come back, that you assumed her moving out was the real deal and you weren't kidding about it. You took her at her word, you accepted her decision whether you liked it or not, and you accepted that she is an adult and in charge of her own life.
Finally, if you need more help, get some short term counseling for yourself or for the family. But you absolutely must give up on the idea that anything you do now can convince her that she's a child who must listen to her mother. That will only backfire. The only hope for her now is real life. She wants it? She's going to get it.
If you are lucky, she will quickly see that this new life is not so ideal, and that maybe her mom knows a thing or two. But you absolutely must work to be the kind of person she can come to without shame or humiliation if she has to admit it's not working for her.