I will ask the obvious first, please don’t get defensive, I am just checking the most obvious issues I personally have come across with weight loss, I an in now way implying that you don’t care for your horse!
1/ Has she had her teeth done recently (last 8 months or so?). Has she been showing any signs of discomfort while riding, ie head shaking, opening her mouth, crossing her jaw, lifting her head and/or tossing it shilw under saddle, reluctance to accpet the bit, changes in eating pattern (takes her longer, stretches her head/neck out while chewing, gums her food) If so, definietly check her mouth, not just her teeth. A mouth ulcer can make some horses reluctant to chew which in turn make can cause a lack of absorbtion in the gut and more whole feed to be passed in manure
2/ Do you deworm regularily with a rotation schedule and/or correct dosage? If not, I’d suggest giving her a different treatment, and/or giving her a double dose of Strongid (not sure what it is there for you). Also, by not giving enough (big mare like her likely 2 tubes) you can create a resistance in the parasites and then they are not affected by the treatments you DO give her. Maybe speak with your vet and see if there is something totally different that you have not used in the preceeding 2 years you can dose her with.
3/Ulcers…this would be my NUMBER ONE suspicion given what you have said. Looking at what you feed her, she is getting a LOT of concentrated feed, is stall kept, which is the number one cause of ulcers in performance horses, even back yard pets who are stabled and grain fed. I would try some sort of ulcer treatment on her, Omeprazole being the number one(GastroGard), Cimetadine or ranitidine (both less effective but a lot cheaper, messier to feed as they are usually pills you have to saok and syringe, they taste horrible). You may have access to something else there, we have very different pharmaceuticals here than you have available in the UE from what I understand. Now, that said, I have rehabbed quite a few ulcer sufferers without drugs, as well as worked in several different race barns dealing with TB’s both with and without ulcers and it can be done. Best thing ever, IMO, feed beet pulp, the shredded stuff in pellets you have to soak in water. Not sure if it’s available to you (pretty sure it is?) but it is one of the SINGLE best things to feed in my opinion (yeah, I know there are tons of studies that say it has no nutritional value etc), but it DOES make a huge difference in how they digest, their water absorbtion and intake, and allows the gut to slow and really get the nutrients of the feed. Second major thing, remove any and all concentrated molasses and oats from her diet; This is totally unscientific, and I don’t know why, but EVERY horse that I have fed a concertrated molasses (powder form or mixed into to C.O.B or sweet feed) to has ended up with ulcers issues. I only ever fed oats to my racehorses/eventers when they are in full training, never my babies or pleasure horses. I worked in a race barn of 45 head, every horse but one got beet pulp (the one was an older finicky gelding who just would not eat it, ever). Not one single horse in our barn was treated for ulcers,(from the nerviest 2 year old twit to the most seasoned guy 8 years old, racing the entire time from 2) EXCEPT the older guy who refused to eat the beet pulp. His diet was 100% the same as the rest of the barn except the beet pulp. He lived on Omeprazole, 2x a day. He was impossible to put weight on, no matter the hay quality he got. I also worked in a barn whose horses were fed a base diet of oats, sweet feed, bran and powdered molasses (we added water to the feeds every night) 26 head, and at some point in the 2 years I was there, EVERY single one was treated for ulcers. Not scientific eveidence at all, but it sure made up my mind!
I have never had to treat an ulcer on one of my own horses, ever, and I have had very little trouble keeping weight on my horses either.
Another non-pharmaceutical option is to give her Aloe Vera juice 2x daily, in her feed or by syringe, 60cc each time. It helps soothe the stomach and heal ulcers.
Not sure that will help you, but if you have any questions, feel free to PM me! Good luck, hard keepers are the most infuriating horses of all 😉