Concentration and alertness

I feel like my concentration is not good at the moment. Maybe I am being too hard on myself as there is so much to try and take in/remember/do on teaching placement. But I do feel I am not as on the ball as I once was and I don't like it! Started on the B12 supplements a few weeks ago but don't always remember to take them. Need to increase water intake as well. Oh, and get some more sleep too.