"I don't know what to say."
God's response is repeated: "I will be with your mouth, and teach you what you are to say.”
Moses didn't have to figure out what words to use. God was going to give him the exact script to deliver. Moses' job was just to repeat what God said.
"They won't believe."
Hard hearts won't respond to words or signs.
The message was very simple: "God says, 'Let My people go that they may worship Me."
The message was very simple. It did not need an eloquent speaker, it needed an obedient repeater.
The truth is that if someone has a stubborn heart toward God, they will not respond to His simple and clear words, no matter how those words are said.