I guess it depends on the location you are in, but saying as a reason to quit, that you feel you do not fit is entirely adequate. I assume there will be a leaving interview? At that point you can provide more detailed summary. One think I would be concerned about (not about quitting, but about you) is you mentioned burn out. Have you sought help? Please do, in any way that makes sense to you and does not impact on your future emplyability (again, this depends on employment law in your jurisdiction).