Vit B12 Deficiency, Brain fog.

Earlier I was having chronic fatigue due to which I could not concentrate on work and my weaknes, fatigue would not cure with sleep so Last year I was diagnosed with B12 deficiency. Now I come to realise that it's the reason why my memory has turned weak, getting difficult to remember things, feel irritated,unable to concentrate, exhausted, In classes my brain gets confused, messed up with even simple quantitative analysis. The doctor had told me to use Vit B12 injection every month I agree intially but later on I miss the dosage.The sister/nurse just pokes the injection as if she is hammering a nail in marble wall.The place where she would inject would be painful for few days. I am a non vegetarian and going by the foods rich in vitamin B12 I eat those with pleasure-Chicken,fish, meat,liver,cheese, dairy even then Its not absorbed by my body. Is there anything on earth to solve this issue*Without injection.