Upper eyelid swollen and red for a month

This issue is despite trying everything under the sun, first OTC antihistamines tablets and eye drops (despite never having an allergy in my life), followed by stronger prescription antihistamines. None of these helped, so my GP prescribed a course of antibiotics which also did not help. I apply cold compresses several times a day which helps get the morning swelling down, but much of the swelling continues. There is no pain. My optician can see no sign of blepharitis or other eye issues.