Add a bucket of haycubes (one In am and one in pm) to her feed schedule and add yeast to her feed
Maybe try getting a vet or a person specialized in horse nutrition to come out and exam your horse.
Try feeding her alfalfa and Amplify. My mom's thoroughbred lost tons of weight and gained it back and more because of amplify
Cocosoya in the powder form is a great supplement. Hay is the biggest weight gainer. Make sure she is getting good quality hay and getting a lot of it. Technically, horses should be eating hay 24/7.
Maybe try adding canola oil to her grain. This worked wonders on my horse when he started to loose weight