What I gather from reading the post history is that the OP already did pass the boards and completed an IM residency. S/he tried to get a cards fellowship and apparently couldn't.
OP, I am sorry, but the previous posters are correct. Medical schools exist to train people to be physicians. You are already a physician, so you cannot go to medical school again. Quite frankly, I don't know why you would want to even if you could. Between the expense you would incur and the sheer boredom you would endure, I think the next 7+ years of your life would be be rather unpleasant!
If you are unhappy in your current career, the only realistic solution is to find a different job, either medical or nonmedical, that you think you will enjoy more. You spoke about doing research in your posts, so that is one option. You could also consider trying to switch specialties (i.e., do a second residency), teaching, practicing in a different environment, industry, government, public health, and probably lots of other things that I'm not thinking of off the top of my head. Other possible options would be getting a second degree like an MPH or PhD, which might be helpful if you want to do public health or research. But regardless, your med school days are definitely done.
Best of luck to you.