Thanks for allowing me to assist you, I am the horse's servant and will do my best to help you with your horse. The one thing that comes to mind is a possible metabolic disorder, has your vet done any bloodwork? A wasting condition like Cushings could be at work. I know the test is not totally without risk and often a horse can be assumed to have a pituitary problem that is affecting metabolism and thus muscle mass. The problem lies in that each horse is unique as to circulating cortisol levels, the dex-supression test has a founder risk, but... the old standby med pergolide is not pricey and I have known vets that will treat these symptoms without subjecting the horse to potential harm from the test. You may want to discuss approaching her condition systemically with her vet. After that, I would check her manure to see if she has any issues (shedding eggs from resistant worms or undigested hay). I know you said she is up to date with her maintenance but it may be worth a second look. On the tangent of her recent herd member loss, a goat or donkey / pony may bring her peace if she is worrying or is emotionally distressed. I wish you the best with your mare, good luck with her.