I don't think I can put a title on this one

My almost 21yo daughter is seeing a 49yo man.  It's wrong on so many levels and she won't listen to me anymore.  She thinks she's in love with him and he calls her, "my love."  I want to puke.  It's not right.  I can't get through to her, I can't talk to her, I can't tell her anything.  I'm afraid if I try, she'll run away.  I'm afraid if she goes out with Sally in a couple weeks that Sally won't be as discreet as she thinks she can be and my daughter will run away thinking that anything she says to Sally will get back to me.  I'm tired of people offering me lame advice.  "Oh, she's got a level head and won't do anything stupid like run to Vegas to get married."  How the hell do they know that?  They don't know my daughter!  She's changed!  She's not the sweet young woman who, a year and a half ago, sat on the edge of her bed crying in my arms telling me how much she did NOT want to get a job and grow up!  She thinks she's grown now, of course, but she has no idea what she's doing or what she's getting into.  And she won't listen.  Write her a letter, I'm told.  I don't know what to say in it.  I could pour my heart out to her but I don't think she'd take it seriously.  She would see it as yet another case of Mom trying to run her life again.  I guess I'm just a horrible horrible mother and should never have had daughters even though I prayed so hard for them.  God only gives me what I need, right?  I needed my daughters, then.  So the hell are they the biggest source of grief in my life right now?  My youngest is a snot and fancies herself a goth just to bug me.  My oldest one thinks I'm out of touch with who she is.  It's so much bullshit I could scream.  I stopped smoking again back in September; but man, I sure could use a long drag now and then.  I sure as hell will take it up in California in a couple weeks.  My daughter is going to kill me slowly and deliberately.  Sometimes, I think she wants to see me hurt.  Sounds selfish, doesn't it?  Maybe.  But I think that's what she wants.  I don't think she sees me as her mom but her enemy that's out to ruin her fun and her life.  I wish she'd just talk to me.  But I can't get her alone anymore.  She's always with the old guy or his 12yo annoying daughter.  Yeah, he's got kids older than she is.  The whole thing just sucks.  I want it gone.