I have taken b12 in the past, but I figure I get enough for the fortified foods I eat and the occasional dairy.
I eat protein powder, mostly for convenience... I'll have 1/3-1/2 a serving before bed and sometimes put it in my coffee if I don't have time for breakfast... usually only if I'm doing a lot of training.
I also go through phases with spirulina (I put it in smoothies and cereal). It stimulates red blood cells, and has some iron. Some studies show it fixes anemia... but it can also block absorption of b12...
It would probably be worth getting some blood tests... get a baseline test when your feeling fine then get another when you start feeling sluggish. No sense waisting the money or dealing with the risks of excessive supplements.