Thanks for this. I can tell you've been through the wringer. Many of us can relate :)
The best natural antihistamine I've experienced is Quercetin. I take 500 mg in the morning. I'd like to take it through the day, but it's stimulating and wrecks my sleep. It doesn't affect everybody that way.
A second natural antihistamine is Ginger. I take 550 mg ginger root twice a day. My only bad experience with it was once the capsule got caught a bit in my throat (didn't drink enough water with it), and I tasted ginger for hours. Luckily, I love ginger.
I've learned a lot from the Low Histamine Chef. She learned to boost nutrition in her diet to keep her mast cells calm.