Did your doctor tell you to take a B12 vitamin to get your levels up? I just read there is an active form of b12 which works well. I think it is called Methylcobalamin. May want to look that up.
You may also want to consider going to an allergist and getting yourself tested for food allergies if the B12 doesn't help. Sometimes we can eat something every day and we don't think that it bothers us, but it can have a cumulative effect. It could even be a preservative or chemical additive in processed foods. Fatigue can be caused by this too.
There was a previous post that gave some other good ideas such as getting tested for mono/EBV, hypothyroidism and adrenal fatigue.
I also just read about two more "alternative" tests. One called SpectraCell that tests for not just vitamin deficiencies in the blood, but in the cells which they claim matters more. And another called Alcat that tests for food and chemical sensitivities. Any one hear if these are actually good?
Good luck. If you don't get answers from one doctor, keep looking for the right one that will help you get better. Don't give up.