Strange experience replenishing b12 levels

Hello Vegan Forum! I have been vegan for 5 years. I never made sure that I consumed adequate levels of b12 - so I became deficient. Over the past year and a half I have been trying to replenish my levels of b12. However, it has some very strange effects on my mind.

Positive: my mind is more agile at quick-thinking activities; i feel SO much happier - sometimes even a feeling of euphoria (but this can be worrying as it seems to make me overoptimistic and not cautious enough).

Negative: Memory is the main thing - I feel like I forget how to do learnt activities - essay writing, spelling - it feels a bit like my brain has been reset. I also sometimes feel quite spaced out.

I’ve done a load of internet searches trying to find any post about a similar experience or some advice - but have had no luck. Please let me know if you have had any strange experiences - even if not the same - when trying to replenish b12 levels. And, of course, any advice would be great!