I was 20 when I had my daughter. Not a planned pregnancy, and I also had always been uninterested in ever having kids. There was a point when I also felt the same way that your daughter does. When my daughter was about 6 months old, I was feeling very overwhelmed, regretful, and really out of my element. I cried for weeks and felt guilty about how I was feeling, fearful that I was just a terrible mom, which made it worse.
Really, the only thing that kept me hanging on and not losing control was the support of my family. My sister and my mother helped me out, and gave me a lot of emotional support when I needed it, without EVER letting me "off the hook". I still changed her, fed her, and took most of the responsibilities for my daughter myself. My mother allowed me one night each week that was a "free" night. I could go out with my friends, stay late, and have fun with people my own age.
I eventually got past that awful hump. My daughter is 14 now, and I have a son as well. I still feel a twinge of guilt for having had those feelings, but in all honesty it was my lack of maturity that was making me feel that way. I needed to grow up quickly, which would not have happened without the love and support of my mom and my sister.
I wish you luck with your situation. It sounds to me like your daughter has a lot on her plate and is feeling overwhelmed. Give her a lot of love and support, but remind her that you have already raised your children.