I just lost an older mare due to her not gaining weight. We had her teeth done...she did have and abssess which we medicated. My vet had me feeding her twice a day a 3 gallon pale of soaked beet pulp drizzled with canola oil, the same pail 3/4 full of a mix of sweet feed, senior feed, oats and vitamin supplements. Plus she had 24/7 access to hay. Although she was nursing a colt she did pick up weight drastically...but got down and we just couldn't get her up. It was horriffic...but she was late 20's to early 30's. We rescued her a year ago and she was already breed. I hope your girl gets better.