You may find some help for the lack of motivation from large doses of B12. According to some swedish researcher, mercury disturbs B12 usage in brains, so that your blood levels doesn't really tell much about the situation in the brains. Therefore looking at some blood values of B12 will not tell if you benefit from taking B12. It also repairs damaged myelin sheaths of neurons.
I only had access to cyanocobalamin which is inferior to methylcobalamin, but it worked for me, and too well perhaps. I got my motivation and a better mood back temporarily, but some unpleasant heart problems as well.
I suppose B12 will liberate lots of mercury, but I cannot tell about methylmercury conversion, some say methylmercury does not cause heart problems, while some others say that I can. I still got the impression that only elemental/inorganic mercury causes heart problems. Andrew Cutler says that methylation of mercury happens in body when taking B12, but not in a significant amount.
Having said that, B12 in doses of 1-2 mg, might work for you as a temporary solution, if you happen to be responsive to B12.