P.S. take a look at the video tells it all
Talk to your doc, but taking a Stress B is probably a good idea. B12 really doesn't necessarily work by speeding the metabolism. Another option are super foods such as spirulina or bee pollen or royal jelly or wheat grass, which give more energy through being very nutritious rather than boosting metabolism, and adaptogens, such as eleuthero, to regulate your adrenals so they work as they should. One of the secrets of East European athletes during their heyday was taking herbs such as eleuthero and rhodiola to tone the adrenals. But you should discuss this with your doc, or a naturopath, and keep your attention on any over-stimulation that might counteract what the beta blocker is trying to slow down. Something else to discuss is the supplement CoQ10, which provides energy to the mitochondria and the heart. Lots of stuff out there to investigate.