Alfalfa is good for putting weight on, if you can't get it, you can feed alfalfa pellets. We usually feed an alfalfa/clover mix.
You may think about stalling and graining her if she doesn't get better, or at least graining her. Go for something with some feed corn in it, it helps with the weight gain too. You definitely did the right thing getting her away from the other horses.
"Sit tall in the saddle, hold your head up high. Keep your eyes fixed where the trail meets the sky. And live like you ain't afraid to die...don't be scared, jut enjoy the ride." - Chris LeDoux