Sounds like stomach ulcers. Can you find a better vet? (By better, I mean one who will actually look into it and try to find the cause and give treatment)
Ditto everyone, especially ^^^^^this.
If she's on grain or any sort of product with grain in it, get her off right away
I would just feed her hay until a DIFFERENT vet can examine her. If you feel she needs something in her feed pan, please just buy a bag of timothy pellets and don't feed her more than an 8 ounce measuring cup at a time.
That is about one pound dry and most certainly add some water to them, to get more liquid moving thru her.
If she is hesitant to eat hay, the timothy pellets would be an excellent way to get forage into her, to keep her digestive system working. You'd have to feed her several times a day, however.
Horses with ulcers or acidosis cannot eat any sort of grain, nor do they seem to be able to handle more than 1 to 1-1/2 pounds of feed at one time.
Yes, ulcers are very common. It is not uncommon to find them in weanlings
Hope this helps and welcome to the forum