When God told Moses to go back to Egypt and tell Pharoah to let the Jews go, Moses replied “I am not a man of words.”  What did he mean by this? Why was he so hesitant to speak to Pharoah?

There is a midrash that teaches us that Moses had a speech impediment and it was therefore a physical ailment.  Rabbi Nathan Lopes Cardozo presents us with a different idea.

It is possible that Moses’ refusal to speak was not born out of a physical speech impediment, but rather from his lack of a clear identity from which he could form a coherent position.  Just because a person recognizes that he is a Jew doesn’t mean that he understands the content of his identity. And such was Moses’ situation, so that he doubted his ability to effectively complete the mission that God gave him.  How can I stand in front of Pharaoh and argue that he would let the Israelites go, if I do not yet fully realize what an Israelite is? No doubt Pharaoh will realize that I am not really, fully one of them. I may belong to their tribe but I have not yet become one of them in spirit, and this will make my argument weak.  My authenticity is not yet well established! A man cannot fight unless he knows what he is fighting for! After all, it is not the physical liberty of the Jews that You, God, most desire. Primarily what You want is to liberate the Israelites spiritually, and so how can I confront Pharaoh when it is not clear to me what that means? (Crisis, Covenant and Creativity by Rabbi Dr. Nathan Lopes Cardozo, pg. 99-100)

Jewish identity is much more than skin deep.  You may be born Jewish or convert into Judaism, but to Jewish identity is something one needs to develop through doing and becoming.