Food allergies caused me to have canker sores and rashes and cramps/diarrhœa that I was plagued with, pretty much since birth, until recent years.
I found it out by trying eliminating various foods for a week, and monitoring the differences.
So … I now (basically) never have soda pop, or chocolate bars, or citrus fruits, bananas, or peanuts, or nuts, or most dairy products, or chicken, or pork, or gluten, or berries.
I now go with a low-salt, low-sugar, low-fat diet.
I also find that eating pretty much the same thing each meal, and about 5 smaller meals every 3-5 hours helps my moods a lot.
I have 6 basic parts: bags of no-name Corn Flakes Cereal, no-name Crisp Rice Cereal, and Regular Cheerios lean beef (out of several days' worth cooked in just in water, no seasoning, on convected oven setting at 350): I squeeze the "juice" from the beef when it's finished cooling, and keep the beef in a big oval left-over plastic container in the freezer-part of my fridge; and keep the beef "juice" in a smaller upright plastic screw-top container in the regular part of the fridge, discarding the hardened lard after it's been in the fridge for like 24 hours, a veggie soup (several days worth in a good-sized crock pot contains 2 lbs. onions, 2 celery stalks, 1 can no-salt tomatoes, topped with a variety of frozen mixed veggies and a small package of okra, covered in water: about 4 hours on high heat): again, I scoop out the solids using a food net into 1 big oval plastic left-over container; and keep the veggie "juice" in a smaller basically-square-bottomed upright plastic container, a rice dish (about 2 days worth is made in a smaller crock pot: containing ½-cup gluten-free brown rice, ½-cup soy beans, ½-cup lentils, ½-cup pot/pearl barley, about 4½ cups water: automatic shut-off is about 45 minutes), bran muffins, and apple-based fruit cups.
Per regular meal, I have about 2 cups of mixed dry cereal, about 3-4 fingers worth of beef, and a proportionate amount of "beef juice"; a heaping small-ladle-worth of veggies, and a proportionate amout of "veggie juice"; a heapingsmall-ladle worth of the rice dish; and about 1/10 of a bran muffin: which all fits nicely in a medium-sized bowl (which would hold 4½-cups if it's just water), and covered with water. My first meal after waking up will have a fruit cup with it, generally to start the meal. Then every second meal after that will have a fruit cup to start it.
It's taken me several years of experimenting and trying tips from others to get to the food prep method (etc.) that I use now.
I don't know who all looks at a post like this, but if anything that I've mentioned here is interesting to you, I'd love to get a PM (since I don't want to hi-jack this Topic). After all, this procedure I've just outlined helps ME stay canker-free, generally speaking.