It has likely to do with hypomethylation issues.
The hallmark characteristic of this is pooling of folate and cobalamin B12.
The main problem with hypomethylation is the inability to breakdown histamine, which accumulates in the body, causing many unwanted symptoms, but wait...it gets worse. Homocysteine levels rise, while the neurological forms of folate and cobalamin become deficient, billions of biochemical processes become impaired to the degree of the hypomethylation, more or less- and in the end, if this does not get resolved, degenerative & chronic disorders develop.
To test for methylcobalamin B12 and methylfolate, you would need an MMA and a Homocysteine CSF test.
Most doctors don't know much about this. The causes are difficult to
verify as they're mainly genetic, but activated by stress and other triggers.
Because conventional doctors will likely keep you in limbo, should you indeed have this, I suggest a trial of methylcobalamin B12 and methylfolate, for a few months, instead of trying to convince your doctors to run the necessary tests.
There are other things you can do as well, but please research this first and then let me know.
This does not exclude the possibility of having other underlying imbalances.