Pony loosing weight

Hi everybody, new person here to say hi, and i have a problem...
My pony is gettin up there in age, ive had her since i was four and back then she was old lol so id say shes somewhere in her forties. Well when spring came she began eating an aweful lot of hay in addition to being out in the feilds all day, but she kept loosing weight well i thought maybe her senior diet oats hay and grass diet wasnt giving her enough nutrition to keep her chubby. So i slightly increased her amount of feed, this worked for some time but now she is loosing weight again, but even worse now. I have no clue what to do, feed more again? or something else?
Thanks for all your input

Hi,
Is she regularly wormed?

yes she is, but do you think it could be worms anyways? like maybe the latest stuff didnt work?

How much does she eat? Have you have the vet out to do bloodwork? What about her teeth? Have they been checked in the last year or so?

have the vet check her, when my first mare got to old the barn manager started putting brewer yeast in with her meals and that helped a bit

she eats the proper amount of senior diet according to her age and weight which is like 2 cups a day and she gets about 3 times that in rolled oats currently, used to be the same amount. The only vet we have around out here lives a LONG ways away and doesnt come to barns to do checkups this far away usually,hes a jerk, absolutely and completely and he charges like over $400 just to look at your horse, not to mention actually giving you a diagnosis and were not made of money out here. No her teeth havent been checked by a professional, only myself after much research.
Brewer Yeast? i'll try that if i can find it. I've been trying to earn money for a checkup, but thanks to child labor laws, its incredibly tough. so far i have $315 but that'll be down soon cuz its comin close to her next worming.