Try megadosing B1 HCL for a month, you might be just super low on it in brain tissues and therefore get mild symptoms that are gradually worsening. You might have slow demyelination - "Thiamine is a cofactor of the very important enzyme Transchetolase of the pentose phosphate shunt that in the brain and in particular in myelin is particularly active." Long-term lack of B1 seems to be pretty common with processed foods diet. B1 is not known to be toxic in any dosage (excess excreted in urine) so it's a safe thing to try.