You may well have been told that your B12 levels are fine Kiki, but the Mayo clinic did a study a while back, which sadly is not widely known about
They looked at symptoms of B12 deficiency in a group of patients, scored them on a scale as to whether or not they had clinical or subclinical deficiency, and then looked at the blood levels. What they found was that the diagnosis of clinical deficiency using symptoms was different to that using blood levels; where blood levels were considered normal, very often there would be subclinical if not full clinical signs of deficiency. (Subclinical means some of the symptoms rather than the majority, or I think maybe sporadic symptoms.)
Typically, the low end of "normal" in real life translated into "deficient". If I had a pound for every time a Crohnie on this forum has complained of symptoms of B12 deficiency having been told that his/her B12 levels are normal/ a little low/ on the low side of normal... Boy, I'd be rich !
The moral of the story is, don't trust a comparative scale that has not been revised in over half a century, when in that field every other definition of recommended daily amount has changed umpteen times. It was created a long time ago, didn't use a lot of patient data, and everyone has just treated it as gospel ever since because no-one had a better idea. Well, now the Mayo knows better, and their GIs routinely adjust for its known failings. Given twenty years, maybe the rest of their profession will have woken up to it.
Me, I take a B12 sublingual daily, and I know it helps; other things like iron levels need to be okay to remove fatigue, and I have found that B5 helps me get rid of that "running up a hill all the time" feeling, but B12 is a must have for me at least.
Don't worry that it is all in your head. If medical testing was that
perfect, we'd all be cured.
There is an old medical saying: "Listen to the patient, (s)he is telling you what the disease is." If you say you are feeling tired, and there is no reason why you would suddenly give up a healthy active life - then if the tests show nothing, they show only that they are themselves deficient. How well you feel and function is the acid test, not some chemical blot on litmus paper.