If she has energy - alfalfa will give her more. Also I have had horses prone to ulcers have difficulty with alfalfa.
You say you feed her flakes of grass hay. What type of grass hay? They are not all treated equally.
I believe in keeping the diet simple and have never had trouble other than weight gain.
I will hang a hay bag (high so horses don’t get feet in them) and fill it with either Orchard grass hay or Timothy grass hay. Keep it filled.
Then for supplements I feed a ration balancer for grass hay diets like Progressive or Buckeye or Stam 30. And throw on a few carrots.
That’s it.
If have horses that get loose manure I give a probiotic.
IMHO you just have to be willing to feed more hay…