Well, for the CFI licenses, especially the initial, there is a bunch to know and learn. That he left you feeling this way is probably good, because you realize how much there is to know, and that you are fallible. It also gives you more compassion for students you may have who don't fare well on a checkride.
Since he passed you, he obviously thought that you were competent to act as a flight instructor. If it left you with this much doubt, either he went at it a little too hard, or you're a good instructor who's concerned for his students and your ability to teach, which will make you learn as much as you can in order to be the best you can be.
My two cents.