When was the mare last wormed? You don’t mention that, so if she has not been wormed recently, that might be worth looking into.
Are you feeding her the same now as she was being feed by her previous owners. Did she take that long to eat when under their care. Most horses gobble their food, and the problem is to slow them down, not speed them up.
Is she an only horse? Horses are by nature herd animals, and some horses just don’t function well as an only horse. However, you don’t have to add a 2nd horse. Many race horses live with, share a stall at home & at the race track with a goat. Goats make good companions, are great at eating weeds horses don’t eat, and generally make good companions.