Help 14 year lod daughter over the edge of reason?

Please help. I have a 14 year old daughter named Katie. Katie's only ignment for homework the whole holiday was to expand on her 200 word essay on Capitol Punnishment. Kate refuses to write the essay. Kate is calling me names arguing with me and refusing to writ the essay. I tried to make a deal with her that I would give her 1/2 hour to start the essay. Kate spoke with her friends on MSN and worte "ESSAY" whan I came back down stairs after the 1/2 hour Kate said there you go I've started the essay. Kate continually downtalks herself saying "I'm not smart, I'm going to fail, I won't get a Job, Knowbody cares, Nobody likes me anyway, I'm just fat and lazy, I wish I were dead" there is nothing I can do to help her. All the time she has this devlish smirk on her face. She said if I wrote this letter to her teachers she would really get mad and then watch what happens. I don't know what to do. On the weekend Katie blue up on me at the mall because I needed to stop and make a phone call. She took off and I couldent find her for hours. Then she made a big deal about wanting a nother new winte coat so to apieze her I went and baught it. We can't afford to be buying her more winter coats. Katie refused to go with me to the mall to take back some cloths that she diden't like from christmas presents. The store won't give me the money back they are insisting on an instore credit. So I want katie eith me to get something she wants. I was right at the store when she took off because I stopped to make a telephone call. I would like to start counceling with Katie, but I don't have custody and I can't make her go even if I did have custody. Please help me to understand why she is treating me and her sister so badle. Katie constantly calls her younger sister a retard. The thing that is so frustrating is that Katie knows she is causing havoc and she does it on purpose. If I had custody I could at least tale her to the doctors and see if there's a drug that will caum her down. Without custody I can't do anything. You get me kicked out