I went through a similar experience- what you describe is pretty much exactly what happened to me when I was a child. My mother had horrible eating and exercise habits, and they reflected very much on me. Now, I will say, though that may have contributed to my weight and general habits as a teenager, there is no way I can blame my weight or habits completely on her- after all, I was the one who chose to continue with those habits as an adult. But, it would have helped me tremendously in making the decision to change my habits if my mother had talked to me about it. I remember her telling me alot that I needed to eat healthier, but never bought the food. To exercise, but never showed me what to do or helped me to do it. I went through alot of the same ridicule and low self-image that your daughter is going through, and it was very painful.
Having said all that, the extremely important thing here is, you realize that what you do and eat are affecting your daughter. That is the biggest help you could provide- knowing that you are contributing to the problem, and acting to correct that. Talk to your daughter openly and honestly about her weight, and ask her what her goals are. Help her to make a plan to eat healthier and exercise more, and you may find that simply helping her to stick to her plan helps you in turn. Take her to the doctor, and get advice on what she can do at her age to safely keep from gaining more weight.
I have a friend with a 15 year old who is 5'6" and weighs 250 pounds. The mother herself is over 400. She knows very well that her daughter is overweight because of her habits, but even though she generally tells her to eat healthy and go outside and be active, she simply doesn't get involved with her enough to provide the proper encouragement. The best thing you could do for your daughter is to actively talk to her, and help her with what's going on. Don't beat yourself up about it- just work with her to change it. :) Good luck, dear.