I had a rescue hard keeper, tried all of the above and nothing helped (he was 21 at the time). Anyway, he got really sick on me and the vet prescribed antibiotics for him. Not only did he snap out of being sick, he started putting that weight on I'd been trying for. Matter of fact I actually had to throttle back his feed for the first time as he was getting chubby.
Called the vet to see what could of changed, he said some older horses that have been neglected will get a low grade infection that their body just can't fight off. If that was the case the antibiotics would of got him over it and allowed him to put on weight. He wasn't sure if that was really the case with him but it was one possibility.
I second getting a fecal done. There is so much resistance in parasites that the AVMA (American Veterinary Medical Association) and the AAEP (American Association of Equine Practitioners) now recommend running a fecal before worming. Seeing if the parasite load is high enough and the species. Then worm with an appropriate wormer. Then run a second fecal 3-4 weeks after worming to see if there is a reduction in population.
If your mare does not like grain (I have one that is not a fan), but loves hay. I would look into a hay stretcher.
I swear by Start-to-Finish Cool Calories 100. You can get it at any TSC store. I've had my mare on it for a while now and her weight gain is simply amazing! She's actually a little chubby when before, at this time of year, I'd be trying to keep her ribs from showing.
I also had my younger, hard keeper gelding on it before I sold him... It turned him almost too fat and I had to cut back his feed before he sold for the first time since I'd owned him!
^^^^ Did cool calories for awhile since its 99.% fat but when adding it up oil is cheaper and 100% fat. CC is basically oil with a inert ingredient to make it grains like sugar. I stopped using it when I noticed the white specks in my horses poop (thinking it was not being digested and therefore wasting my money) and also the cost.