From what I personally experienced, a low B12 can really mess with the CNS, and a person's persception of "pain". In other words, it'll feel like you're falling apart, and that you're walking on broken feet. Most folks are naturally low on vitamin D (which the human body "makes" with sun exposure), and need to "supplement", prefereably with diet. Realistically, mineral supplements are usually necessary. Neither should be taken without doctor supervision, since going "over" or "high" with the levels can cause even more problems.
Now, when you use the term "retarded" (which just means "delayed" or "slowed"), are you referring to feeling like you're losing your faculties, or your ability to cope? Losing your "grip on reality"? I ask this because I wonder about your thyroid levels, and if they were checked? Not just the TSH level, but T3, T4, etc.? Hypothyroidism can *really* cause a person issues, and combined with low B12, you can end up wondering which way is up in your life, with symptoms like you describe... First-hand experience here with that. The thyroid regulates just about everything in your body...
btw, Welcome to WHL, Shaea. Browse around and read the "stickies" at the top of each section for some useful info.
"There but for the grace of God, go I."
"... His mercy endureth for ever."