Why did Moses feel unable to approach Pharaoh after 40 years?
After four decades away, he had lost his Egyptian polish, and he told the Lord he was not eloquent but slow of speech and slow of tongue. His earlier self-prepared attempt had been rejected by Israel and was not of God, so he felt unable to approach Pharaoh.
Les explains that after 40 years Moses did not feel he could approach Pharaoh. He had lost the polish he once had when he first went out to deliver Israel, and although he had been 40 years in Pharaoh's palace and had all Egypt's wisdom, polish, charisma, and power, Israel had rejected him. God was not in that self-prepared effort, and God cannot use someone who is self-prepared or prepared by the world. In Exodus 4 Moses says he is not eloquent, slow of speech and slow of tongue. The Lord answers that He made man's mouth and will be with Moses' mouth and teach him what to say. God had already given signs—the rod becoming a serpent and the leprous hand healed—to prove He meant what He said.