I suggest first having a vet check to Rule out ulcers. some people have their horse scoped. others just try omeprazole for a month. If your horse has ulcers, try to feed frequently so there is always something in the tummy to digest. also you can use U gard liquid, albout 12 ccs just before riding. any travel, such as to a show, can cause an ulcer. they are far more common than we expect, and are easy to prevent once you are educated about them. keep omeprazole on hand.
If it is not ulcers, then you could try a good accupressure or accupuncture evaluation. In my experience it is always a good idea to have a vet rule out any kind of pain problem and treat it. then look at your saddle….try every way to make sure your horse is not in pain, or anticipating pain, when you prepare to ride.