I, too, had a mare that looked similar to the OP's. She was still eating and drinking well. My final straw was when she came home from my trainer's looking stick thin. Vet drew blood and she had a bacterial infection. She had it for months, and I got lucky she didn't die on me from me not being able to find what was wrong. After 10 days of antibiotics, she was all better and rapidly put on weight after that.
I'd suggest a blood panel done on her, because you just never know.
I don't see a roach back, I see a horse that needs a good 200 pounds put on and her topline built up.
Good luck, she sure is pretty!