Thoughts other than food............which she may need more of/different, etc. Yeah, they have preferences
:
1. Have you had her teeth checked RECENTLY?? At 19 she may well be starting to lose some and they may be loose, long, hooked, etc. This will cause issues. Especially since she no longer prefers hay.
I am assuming that she is being fed where there is no challenge for "her share" of the feed from others. If so, then feed her so that no one can share hers. These older ones will eat slower, normally.
2. You don't say how long you have had her. SO, if a fairly new to you and her surroundings mare, she may be having some ulceration from "stress". I know, I know, it doesn't seem possible but, oldies don't like a lot of changes. Let's say that most of the time some ulcer meds will cause no harm but, if this is the cause, you will see attitude and eating improvement within a day or two. For ME that is a sign that there was some tummy issues and I keep administering until heal time has been given.
I am not a vet but, a long time horse(s) owner and caregiver. If she were mine and still "picking" at feed, these are the areas I would investigate. These things not giving results, I'd have the vet run bloodwork to be sure there wasn't something else going on that needed attention --- infection, anemia, worms, etc.
Keep us updated -- we care and want to know what may have helped/been the problem. We learn.