I too have a mare who is a hard keeper, eats slowly and would more often than not walk away from her grain. I had the vet out several times, had her teeth floated, Power Packed her, added supplements, gave her hundreds of dollars worth of Gastro Guard, ect.
I fed a high quality extruded sweet feed along with soaked beet pulp and alfalfa cubes. She had as much alfalfa as she could eat as well as good pasture.
Long story short, after a lot of trial and error, I switched her to a high quality low starch pellet. That has made all the difference. I still feed soaked beet pulp (but without molasses much to the disappointment of my other horse) and soaked alfalfa cubes, and I include rice bran and a cup or two of Calf Manna. I also added peanut hay and she has access to plenty. She still eats more slowly than my other horse but not anywhere as slowly as she used to. She has put on and maintained weight.
I hope this is helpful – good luck!