Sever fatigue AFTER taking methylcobalamin

Despite my serum b vitamines being at the high end of normale range I decided to trial methlcobalamin.

My symptoms are a sore and sensitive tongue, muscle twitching and worrying forgetfulness (I'm 37). Other minor symptoms of weak nails and hair loss.

I wanted to feel better straight away, so started with 5000mg sublingual methylcobalamin from a quality make of vitamins.

I noticed a strong fatigue reaction within a few hours, and after 10 days have had to stop because of extreme fatigue. I can't even focus my eyes to read!

After stopping the fatigue went within a day.
None of my other symptoms had yet improved.
I'm convinced I need b12, so would like to continue to trial it. Am I just taking too much? Or do I need to combine it with something else?
(I already take D3 and magnesium)