I can't speak for the B12 deficiency but I feel the same way with my vitamin D deficiency. It's gotten a bit better but I'm still fatigued more often than not. My doctor prescribed me a supplement that has both vitamin D and K2 in it, he said something along the lines of the K2 helping to absorb the vitamin D I think, so maybe you can try that? I don't know what your dose is but if you're very deficient you may not have one high enough. My doctor tried to reduce my dose and I found it difficult not to fall asleep in the middle of the day which was a massive step back