First, what an unbelievably arrogant thing to say, to suppose that you know someone better than they know themselves, leaving aside the fact that that’s a completely unprovable statement. As Miss Manners points out, being related to someone is not a blank check to say things that you would never say to someone who wasn’t a relative. How would you take it if I came up to you on the golf course and said not that I disagreed with you, but that I didn’t believe you agreed with you? You’d have a think about my sanity, for starters, and you probably wouldn’t call me for drinks until I’d stopped saying garbage like that.
Second, what an incredibly self-centered thing to say. It has nothing to do with your child’s development of a sense of self (or, incidentally, a constructive relationship with you) and everything to do with your insistence on your child being who you’ve decided you’d like her to be. It’s more about your insecurity than about your child and, as such, it’s pointless to say to her, if not outright damaging.
Even if you are 100% convinced you are right*, even if your child is confused or still sorting out who she will be, it will only add to her burden and confusion to have you or anyone else projecting onto her and judging her simultaneously. (Even if what you’re projecting onto her is herself: a 19 year old isn’t likely to enjoy being told who she is based on who she was at 12. Or who you thought she was at 12.) And, to boot, you’ve effectively cut yourself out as a resource for her at the same time by proving you won’t listen to her or trust what she says, even about herself. You have to ask yourself what exactly you were hoping to accomplish by saying something like that.
Bottom line: it’s not useful to her. You might think it’s useful to you, but you’re wrong there too: you’ve alienated her by effectively saying that you like your projected reality more than you like her. And you’ve done it to ease your own issues at her expense. Your deal is your deal. If it’s a mess, share it with your friends and your therapist, not your kids.
There, I’m done.
*If your kid is saying things like, “I’m a heroin addict and worthless,” you should feel free to say nice things in response, but I still don’t think direct contradiction is the right route. “I know you feel terrible. I love you. Let’s turn this around,” is different from, “You are not!” Which, if the kid has a needle in her arm, would be technically incorrect anyway.