I think that alot of us here can relate to how you are feeling. I know from my own experiences that I have been where you are and I understand the deep, dark moments. There have been days where I tell my son that I love him, but I wonder to myself, "Do I REALLY love him, or am I just saying that because it's what he needs to hear?"
Was she always a tough kid, or is this something that has started at the teenage years and puberty have set in? Has she been evaluated by anyone? Does she take any medications?