First, I can relate. My dd has been severely ill for years, and she has had stomach pains for, well, it seems like forever.
You don't need to worry about horse allergies and gelatin. Horses aren't used in the production of gelatin - cow and pigs are.
What has helped my dd:
A low residue food plan. Residue is not the same as fiber. It's quite simple to follow, and there are lots of helpful guides online. PM me if you need more info. Basically, vegetables are peeled and cooked until tender. Raw foods are avoided, as are seeds and nuts. Chewy, stringy foods are avoided (beef jerky, brisket, celery, etc).
Look into the Whole30 plan and commit to following it religiously for at least 30 days. 45 to 60 is best. It eliminates grains, dairy, sugar, etc. There are Facebook support and recipe groups, and the plan is amazing. There's nothing to purchase or sign up for. It's just really clean simple eating. The basic premise is that food must be pure, in it's original form, recognizable (no zucchini "cookies" or vegetables made to look like pancakes). And yes, eliminate corn, corn syrup, cornstarch, etc. Look online for a list of corn products.
Switch from any sugar, no matter how "raw" or "natural" to only organic maple syrup and/or pure raw local unfiltered honey, and go easy on both of them. Absolutely no stevia or agave. They're highly processed and many people react to them.
Another idea: does your daughter eat or drink ANYTHING that is "diet", "sugar-free", or artificially sweetened? That includes gum, soda, sugar-free drinks, fat-free stuff, anything advertised as "sugar free". And it includes the low calorie versions of regular foods, like low-calorie sour cream, for example. If she does, go cold turkey. My dd is severely intolerant, though not technically allergic, to sorbitol, erythritol, mannitol, isotol, aspertame, etc. They give her an instant stomach ache. If your daughter likes a glass of juice, switch to white grape juice with no other ingredients (Welch's makes one). Sorbitol, etc, is naturally found in apples, cherries, purple grapes and a couple other foods.
Going back to the sour cream example, look at the ingredient list on a tub of Daisy low-fat sour cream vs the list on the regular full fat tub. There is a long list of stuff in the low-fat, vs 2 ingredients in the full fat. Use the purest ingredients you can. Buy the full fat sour cream for the tacos, but only use a little. Use real parmigiano cheese, not the green can. Keep your foods' ingredient list to a minimum.
And try to avoid carrageenan, xanthan gum, and other gums. If you drink commercial almond milk or soy milk, look to see if they contain carrageenan or gums. Make your own (they're easy to make). My dd also gets stomach aches, bloating, and pain from carrageenans and gums. Gluten-free organic oats make a delicious "milk". You just soak the oats, drain, throw them in a blender with water, and strain. I bought a milk-straining bag for just a couple dollars on Amazon.
It's good that you're keeping a food diary, but make sure to include time of day, and what activity is going on when the food was eaten. Include fluids as well.
Don't give up!
