I can tell you that I know your pain. I have had hives for almost 2 years now. Never before been allergic to a thing and suddenly I was breaking out all over. It has been the most excruciating, horrible experience while at the same time helping me mature and grow so much in many areas. But enough of that. :) Suffice it to say, this "diagnosis" affects so much more than just the physical.
Yeah, all my tests came back negative as well and none of the Drs I went to liked the idea of me being on Prednisone long-term (although initially that was the only thing that had any significant effect). Allergy shots weren't working, and I had to find something that would let me live a somewhat normal life.
After trying every diet, home remedy, and medication in the book, I gave up and went to an alternative doctor. Believe me, I'm not the type of person to do this, but when you've been sick with this for 2 years straight with no relief you're willing to try anything, right? That's what I figured anyways.
I wish I could say it gets easier. Unfortunately, there are nights that I wish God would take me home so I wouldn't have to suffer like this anymore. Anybody who thinks this is over-dramatic hasn't experienced the sleepless nights, endless itching, and horribly disfiguring swelling. However, I have hope. Definitely a weak and sometimes wavering hope, but hope nonetheless. My alternative doctor told me I have leaky gut syndrome and has been able to help some, and we are working towards a cure. Maybe unrealistic, but to be fair I have seen more improvement with him than anybody else. The healing process is bumpy with lots of setbacks even when I am doing everything right. The only thing that gets me through is the knowledge that God has a plan for me through all of this, and He cares.
Hope this helps. Don't give up!