I have had hives for several years now and no one can give me a diagnosis. What could it be? I'm not allergic to any foods

Autoimmune Urticaria. About 50% of chronic hives are caused by autoimmune antibodies. No cure. But use high dose antihistamines to prevent hives.
It's a hard one. Unfortunately, the cause of hives often cannot be found. Simple things like exposure to cold air may trigger that. I've heard of case of hives caused by thyroid dysfunction. I personally believe there may be a hormonal problem that is causing that, but what hormone is doing that is hard to tell. Sometimes empiric treatment may be attempted to find a reliever. I truly sympathize your frustration.