I think this is quite common for anyone attending law school or any new commitment. Whether to attend law school or not is a life altering decision and you there is no guarantee it could go great it could go terribly. When I was a 0L I can tell you I was freaking out the summer before I started it was a mixture of excitement, anxiety, hope, dread, and everything in between. However, most people in their mid twenties have the feeling of "what I am doing with my life" and if you attend law school you are taking a significant step in one direction it is a 3 year 100,000 commitment that will change your life.
It does sound like you have done a good amount of research into it and worked in a law firm and were not scared off. As for having a possible false vision you might have one, but there is no exact picture to being a lawyer it is a diverse field much like anything else really.
As for the JD/MBA what do you want to do with both of those? You shouldn't simply get more education because you don't know what to do and personally I think a law degree is far more valuable than an MBA, but I could be biased. You need to go to law school to be a lawyer you don't need an MBA to be a businessman.
I am sure you would like to hear everything will work out and this is a great decision or maybe it is horrible and you should not go, but this is your life and your decision. There is no way to know if law school is the right decision and like any big decision there is a bit of uncertainty. Whatever you decide I wish you good luck.