Pediatric Autoimmune Neuropsychiatric Disorders (PANDAS/PANS)
My son had a similar situation. Research PANDAS/PANS. This is when an infection (virus, strep, Lyme, mold, or other issue) cause major behavioral regression. You may need to find a doctor knowledgeable about PANS/PANDAS to help you. My son is doing much better since getting a diagnosis!
I can feel your pain. I would definitely recommend finding an eating disorder specialist immediately and also visit her physician if you haven't already. I was in a very similar situation with my daughter about a year ago (she was a few years younger than your daughter though). A previously picky eater, she rather suddenly became depressed and very avoidant of eating, with very OCD-like rules about food. We tried regular talk therapy for awhile and it was useless. The therapist claimed I was "giving her too much leeway" with food and that she must be acting out about some deep seated trauma (typical mother-blaming stuff!). Eventually she lost so much weight we ended up in the eating disorder unit at Stanford for a week. In her case, it turned out she was suffering from something called PANDAS and was treated with a combination of anti-depressants, anti-inflammatories, antibiotics and therapy.
Now, that might not be the case with your daughter but no matter the underlying cause, I can highly recommend Elizabeth Burns Kramer in Rockridge, who specializes in eating disorders. http://www.elizabethburnskramer.com/ Elizabeth worked with us after my daughter came out of hospital and it made all the difference. She is super warm and kind and she helped us come up with some really creative strategies - not just talking! She is young too, and I think she would be relatable for teens. I can also recommend the book about childhood eating disorders called "Giving Food a Chance" by Julie O' Toole. It contains a ton of useful information and it reassured me that yes, my child was suffering from a eating disorder, and no, it wasn't my fault or hers! Best of luck to you and please feel free to reach out to me directly.