Hi , I too have a b12 deficiency, I have Addison pernicious anemia, which basically means , my body doesn't absorb b12 , it caused me to get numbness and tingling in legs feet and hands , extremely tired and feel very low .I get heart palpitations and irregular heart beat ,  It is hard to diagnose until you have tried iron medication, and nothing works , they took me into hospital for tests , then they found the problem, I had b12 injections every week to build up . I now get them every three month at the doctors. They make a massive difference. The doctor told me , years and years ago , people died , because not enough iron for their organs , that's what pernicious means . Deadly, it sounds scarier than it is , they will hopefully get you sorted . Hope this helps .   Try not to over think . I know that's easy to say , but stay positive and take care . Keep in touch if it helps to talk .