Moshe says to Hashem "Who am I to redeem the Jews from Egypt" to which Hashem answers "I will be with you" [Shmos 3/ 11,12].
I have a "better" answer. Hashem should have said to Moshe "NO MOSHE, you're the BEST!!"
The answer is a fundamental lesson in good communication. When someone says something you disagree with DON'T contradict the other person. It just creates resistance and a feeling of not being understood. Rather, go along with him.
Moshe felt that he wasn't worthy. Hashem, in essence, answered "Moshe, you're right, maybe you really aren't worthy. But still, you will succeed because I am with you." This way Hashem achieved his goal of having Moshe go while at the same time validating his feelings.
[Based on the Sefer Zikaron Zos of the Chozeh of Lublin page 41]