I, similarly, am very worried about my DD18, also, and it's over an abusive relationship. DD is in her first "serious" relationship and it turns out the boy is verbally and emotionally abusive when he is drunk, lots of yelling and screaming and guilt (yes he is only 18... college+dorms+frats=drinking). Borderline-bad stuff, but enough so that she finally read him the riot act: At the end of the semester she moved to a friend's place (so he couldn't find her) and told him he had the summer to quit drinking and shape up, or they were done. He checked himself into rehab, joined AA, got counseling. She was thrilled but cautious; we (DH and I) were skeptical, at best. He's very clingy, refers to himself as "your future husband", posts drippy garbage on FB for her, etc. Now he tells her that his mom told the pharmacy to stop filling his anti-depressant 'script so he's not taking it (LIE!!!). He's also told his entire family that she is "The One" and that they will get married and stay together forever. DD and I have had serious talks about him/them this summer; she feels he's too clingy, doesn't think he is the one at ALL (and she has Things she wants to DO with her life before thinking about settling down), but she loves him and wants to give him a chance to do what's right (be her bf without drinking and abuse), knows he can... but she still doesn't see him in her future, long-term, but she's just not ready to dump him without giving him a chance, because she gave him an ultimatum and he upheld his end, so she needs to uphold hers. The really "funny" thing is that DD doesn't see this guy in her future, she just isn't ready to go through what it'll take to get rid of him.
I worry about her safety, her sanity, her ability to be herself. I think she did an incredibly difficult, strong thing last semester by recognizing the bad relationship and telling him it's something she will no longer tolerate. She's a smart girl and has even said she knows she cannot change him, HE has to change him. She's even said she is embarrassed that she tolerated last semester's crap for so long. I mostly worry that my "people-pleaser" will stay in a crappy relationship because it's more important to her not to hurt him than it is to keep herself happy. One consolation for me is that she has a good counselor who seems to be helping her in several different areas, so I think she can help DD stay strong in this one, too... I HOPE. Fortunately school is here in town, so if it gets bad (he has already said "what'll I do without you, I think i'd kill myself if I lost you," etc.) she can move away from him but still stay in school. I know there isn't anything I can do except stay strong for her (sorry PP, I could never give up on my dd), listen and offer advice when asked, etc... but it keeps me up at night, occupies my thoughts all the time, and makes me very, very anxious for my DD.
ETA: Just read Christine's post. Earlier this summer DD was supposed to go to the bf's for 3 weeks and ended up being pretty scared and insistent about NOT going, and asked me to NOT let her go so he would "accept" it. (He tried to argue with me over the phone about DD doing what she wants as she is an adult now and I as her mom just have to accept that and get over it... yeah, riiiiiight...). DD ended up compromising and going for 5 nights. Christine's post gave me comfort from hearing that teens DO appreciate the parental support in a bad situation... perhaps we are on the right road, letting DD manage her relationship but keeping her grounded in reality and supported. (Needless to say I am hoping he hits the bottle as soon as he moves into the frat.... DD insists that if he goes back to drinking, they are FINISHED. It'd make it all so much easier in the long run...)