I wish I had suggestions for you. I started reading and was like "Vet!" and then read that you had not only done that, you'd also tried more than one solution from the vet.
Is it possible her mouth is in pain? I feel like the vet would have checked, but is there anything in her mouth that would make it hurt when she ate? How long has this lasted?
I wish you the best, I am sure this is really scary.