Hello dear and welcome to the medhelp forum. B12 deficiency can manifest as weakness, tiredness, pale skin and easy bruising, weight loss due to stomach upset and diarrhea or constipation. Uncorrected this deficiency may lead to damaged nerve cells and neuropathy, depression and confusion. Usually this can be treated by changing dietary habits to include animal products etc. any nerve damage due to deficiency could be irreversible. Discuss with your neurologist in detail the status of your deficiency. Wishing you all the best.
I had severe vitamin B12 deficiency and have permanent nerve damage. I noticed my memory improving after a month of B12 treatment. I can't say when all my symptoms went away but it would take a year for my nerve damage to improve as much as it was ever going to improve.