Jeanne. I sympathise. I am also having problems with my eldest DD at present, who is a similar age to yours, although I believe a lot of the problems are caused by her controlling BF. My other DC say that this is not the case and that DD1 is responsible for her behaviour, and that I am deluding myself. I suspect they may be right, but I love DD1 passionately as I do all my DC. It's unconditional love, of course.
However, I feel that I am also allowing my relationship with her and its ups and downs to influence my life unduly, which is not fair on DH or my other DC. I was quite depressed and have been in counselling which is helping.
I think it is right that our DC need to separate from us and find their own identity, and that we should not judge. It is for them to make their own way in life. In my DD's case, she and BF - both highly educated and DD with a professional cookery qualification - have not worked since leaving uni three years ago. It is a lifestyle choice as they feel that they are different from other people and don't want to be tied by jobs or mortgages. They have been claiming benefits and have made it clear that I will not see DD unless I comply with their demands for money.
I wonder what I have done wrong as a parent, but my other DC could not be more different, hard-working, motivated and, above all, they are kind, they love me, they genuinely care about me.
I have always thought of myself as a tough person, stand up for myself etc, but this has floored me. My instinct is to fight but I don't want to make a bad situation worse and I hope that she will mature and change, and of course I love her. Only those whom we love can truly hurt us.