Please don't lose hope, I also was an overweight child (now in my mid 20's)and although I wasn't severely overweight I can relate to a lot of the symptoms you describe in your daughter, trying every diet w/o success, depression, negativity. After many years of studying natural medecine, I discovered that I had low-thyroid function after fixing my endocrine system, my metabolism and outlook on life have changed 100%, nowadays I am thin.
What I would recommend you do is take your daughter to a physician who will run a endincrinology testing on your daughter, because if she is that overweight she most likely has an issue with her glands. But be careful because many doctors are limited in their diagnosis (I was improperly diagnosed as having a healthy thyroid function for many years until I saw the correct doctor). And many doctors look to prescribe synthetic thyroid meds, which often are of little help. If she does have low-thyroid, Armour thyroid is the best, but many doctors will not prescribe this. I recommend you do a google search and inform yourself since there is too much information on Armour and properly diagnosing the endocrine system, to put in one email. If she is hypothyroid, here is a good site I recommend: http://www.stopthethyroidmadness.com/.
Also, a supplement that has worked wonders for me is maca root. I began taking this as an adult, but since she is still a developing teenager, please inform yourself in regards to this supplement before giving it to her, since I assume it would be fine for her but cannot say from personal experience. However, maca root is a natural whole food supplement that I take in powder form and it is know for balancing one;s endocrine system and giving one extra energy.
She of course needs to focus on eating healthy and staying away from Sugar and processed foods, but if her endocrine system is out of balance oftentimes it will be that much difficult for her to do this, since she will experience hunger and Sugar cravings, so you need to fix the root cause of her problem.
Another thing is that it is unfortunate, but other kids and society at large look down on overweight people and this is probably affecting your daughter self-esteem. Get her enrolled in activities that will boost her esteem, such as art class, or any other area she is interested in. She will understand that she can succeed at things and this will develop a positive self-image beyond her appearance. It is important to work on her psychological health and self-image and let her know that she is a beautiful and wonderful person no matter what her weight!
I commend you for caring enough about your daughter to reach out to others and try and explore alternative methods. I hope your daughter is able to find some help! Best of luck!!