So sorry to hear of your tooth troubles.
I work for an endodontist and you story is not the norm at all!

Has anyone has checked your occlusion? Surely they have.

I believe you should get a third opinion from another endodontist. There are a number of things that it could be - the occlusion, a cracked tooth - which by the way, did the endodontist use a microscope?, there could be an auxillary canal, the canals could be under-filled.

A good endodontist should be able to give you at least an idea of what's going on and you should not have continued pain.

I so hope you are able to find a resolution and save this tooth.