21 year old ISTJ with ADD - What career? Am I in the right line of work?

I think your focusing to intently on the ISTJ thing. I believe in all that too but you have to keep it into perspective...it's a tool, one of many, but not a definitive answer. The whole ISTJ thing does not take into account ADD, depression, anxiety, etc. which further complicate things with regards to combining personality with biology. Relax, okay...you have a lot of life ahead. I've changed careers 6 times ...completely different fields. Two ways to approach that 1)Just go with whatever you are pionate about(will cause you to do better but also take away from your personal life) 2) View your career as a job...9-5 or whatever, that might help your personal life tremendously. It's a tough thing to balance when you have ADD, but very doable...give it some thought with career vs home life balance....