My opinion is this.
Horses are designed to be eating continuosly. It is bad for their gut if they aren't. All my horses are at pasture 24/7, but this applies even if they were stalled. I can reduce the quality of the forage or the amount of sugars in the forage but I won't reduce the amount. If a horse is too fat on simple forage, then that fat needs to be taken care of with excercise, not reducing the forage.
Extra feed is only given when the horse needs more calories than they are getting from their forage. I.e. they are not maintaining weight or are lacking in energy. I add calories in the form of feeds low in NSC's - So NO grains as horses guts weren't designed to digest grains (Though oats are the best of a bad bunch). I use fats to supply calories as opposed to sugars - Much better digested and utilised. So things like Copra meal, Soybean meal, Lupins, Oils. I always feed these mixed with a forage - I use Speedi-beet - To minimise any gut overload.
Vits/mins are managed with either free choice blocks or a ration balancer. Mine aren't being fed at the moment so they have a free choice mineral block.
I tend to only feed calories when i'm competeing and just add more hay for their daily ration. because I feed fats and not sugars and mix those in with a forage (Speedi-beet) along with plenty of hay when we are away it doesn't cause digestive upset.
So if I were you, I would get her on free choice hay if you can, drop the oats and stick with a ration balancer or your vit/min mix. You can mix the vit/min mix with a small handful of Speedi-beet or smething similar so she can eat it. I know you are eventing so if I were you I would be feeding her fats as energy at competitions mixed with a lot of forage.