I'm so sorry to hear about your bad experiences with medical doctors.
I usually just don't go back if they don't believe me or take me seriously. I find someone that does take me seriously, and believes me.
You are not wasting anybody's time!!!

I'm sure you've tried elimination diets. And I hate to be annoying and suggest this but...
Did you try very careful long term elimination diet for EVERYTHING?
I found that milk was mysteriously causing all of my skin problems (rash, eczema, and acne) but not lactose like everybody else, mostly whey protein and a little issue with casein proteins - unfortunately it's often last on the ingredient list, or hidden (sometimes even called "natural flavoring"). There's also a blood test for allergies, but that one is fairly expensive. (Maybe you've tried that already?)

I still have obvious/unobscurable facial scarring from acne that only persisted because my doctor did not believe diet/lifestyle could be causing my symptoms (because of their confined locales) and instead tried to medicate the problem away with steroid creams, antibiotics, etc. Nothing worked. I went to a new doctor, who had me do the allergy blood test, and we found an issue with milk proteins. When I cut out all the milk in my diet, all my skin problems went away within a few months.

I hope you find a solution soon. I hope you find a doctor that believes you and tries as hard as they can to think of every reason for the problem, instead of trying to simply suppress symptoms with creams and pills. You deserve better!