My stoma itself has never hurt, but the skin around it has caused me to lose sleep a number of times. I feel for you! I have started showering "naked" on occasion, meaning without a bag or wafer. Mine is an ileostomy which, as I understand it, is more active than a colostomy, so I wait until long after a meal before doing so.
I prepare a new wafer so I can put it on as soon as I finish showering. While in the shower, I remove the wafer and use an adhesive remover wipe, and let the water run over my belly. Then I use a foaming soap that is actually part of a facial skincare line called Vichy. It's for sensitive skin - no perfumes, and I swear I can feel my skin relax instantly. (I've used Vichy for years and love it.)
I would try to go wafer-free for as long as you can - even if it means sitting in the shower/tub for an hour. Let your skin dry completely before applying any products, and then apply them in stages, to avoid irritating your skin.
I've also found that letting out a good scream in frustration can be helpful. 🙂 I get so tense when my skin is irritated that I can feel it all over me. Slow, deep breathing is relaxing. As is Lavender essential oil, it's a stress reliever. Sprinkle a couple of drops in your bath while you sit on the edge of the tub with your feet in the water, (don't sit in the lavender bath without a wafer!) and maybe some Epsom salts. I find that if the rest of my body is calm, I can deal with skin irritation a lot more easily.
I hope that helps, Verdun. Hugs to you.