i remember after the grammar exam part of my assessment to become a tefl teacher talking to the director of the school. i had similar worries like yourself. he and my tutuor both agree that teaching is through practise and "love".
the grammar aspect is quite logical. rather that thinking about how much you know, indeed, how little you know, focus on the learning and your student. and if it is a grammar aspect that is new to you, enjoy the new learning.
the number one thing is to exude confidence, and if you do not fully know a student's question, be honest (well, not overly-honest) and tell him or her that you will return to her with the answer.
it is a good job, you will never know everything, but neither will anybody. so enjoy!