We have tried all sorts of rewards and punishments for years, provided support and tutoring, and had numerous meetings with teachers and school counselors. I am at my wits' end! She is not an only child, I work full-time, and I need to spend a great deal of time and energy with her. Please help!
If she has been in therapy for two years and there is no change, it is time to find a professional counselor specializing in adolescent treatment. I would suggest a complete assessment by the counselor, but I surmise that she might need to be placed in a treatment center for a while where trained professionals can work with her and your family.
Another possible plan would be to check with your state's Human Services Department to see if they have a program for out-of-control teens. Such programs usually include parenting-skills classes and counselors who visit the home to help families set structure. At the very least, I encourage you to enroll in parenting skills classes and/or counseling for you and your husband. You need support in restructuring the boundaries necessary for your daughter to feel good about herself, for you and your husband to maintain your relationship, and for your other children to learn and experience a stable family life. Also try to read all you can.
I know that you love your daughter, but I urge you to take the focus away from her by getting assistance for yourself and the rest of your family. Any of the above plans can work, but they all take time -- so you have to be in this for the long haul. It's worth it for your own peace of mind and for the happiness of everyone.