Cetirizine really won't prevent you from reacting to a food allergen-- especially not if you're actually EATING the food (not just accidentally getting a little bit of it). Anything that you've eaten WITHOUT reacting to? Yeah, I'd call that "not allergic to this food."
Why do I think that is important? Because you have to have SOMEWHERE to start with an elimination diet-- and the best place to start is with whole, minimally processed foods that you can prepare yourself and which are reasonably easy to obtain. Then you start with those, and start branching out-- SLOWLY, and taking notes about everything you eat, touch, use in personal products, etc. Know that MOST food allergy reactions start within an HOUR of eating the food. And when I say "most" here I mean about 95% of them. Yes, it's
possible to react 2 or 3 hours later, but it's quite unusual. Outside of a 4hr window, the percentage is TINY-- far less than 1% of food allergic reactions occur that way. This is why the first thing that doesn't fit is "waking up with" symptoms. That's much more indicative of some OTHER kind of trigger for the reaction.
Truly-- I think you ought to find a different doctor. I realize that may be difficult, but the one that you saw seems to have given you more
difficulty and reduce quality of life than answers. It doesn't seem entirely plausible that you have
food allergies and not a significant allergy to something environmental. Given the severity of your past reaction, it's pretty important to KNOW if you happen to have an insect allergy at the bottom of this.
Your symptoms sound not unlike what I experience when I'm exposed to high doses of cockroach/dust mite residue. MAJOR hive-o-rama and itchy-itchy-itchy. Now, I later developed a shellfish (crustacean) allergy, and I have always had a wasp allergy... and I certainly cannot easily go a day without my beloved cetirizine!!