Although Moses had a speech problem, and lacked confidence because of it, God had a plan that would go beyond any physical, or emotional issue.We all have different "reasons" why God simply cannot use us. God was calling Moses to Leadership, to bring the children of Israel out of the oppression from Egypt. At first Moses questioned God and asked "Who am I to go to the Pharaoh, and that I should bring the children of Israel out of Egypt?"(Exodus 3:11) Gods answer was simple and one we should remember. In the next verse He says, "I will certainly be with you..."
The next verses are concerns or "reasons" why God couldn't use Moses (in his mind).
Exodus 4:1- Then Mosses answered and said, "But suppose they will not believe me or listen to my voice; suppose they say, The Lord has not appeared to you."
Exodus 4:10-Then Moses said to the Lord, "O my Lord, I am not eloquent, neither before nor since You have spoken to Your servant; but I am slow of speech and slow of tongue."
Here it is easy to see how our human minds work. We have these thoughts where we convince ourselves we are somehow disabled and not useable, But God see it so differently. Remember the verse Exodus 3:12. "I will certainly be with you..."
I know for myself I get to this place of "God can't use me for this or that..." However, we know from the story of Moses that reasons, and excuses are simply just not the truth, not Gods truth. God see's so much farther then we can, so why not just trust? Why not put the lies away that I cannot be used because I am too young, or insignificant in some way. I believe if God give a vision, or a word, or knowledge, or insight, or a gift to do His work, then it is a job given, and He is saying "I will certainly be with you". He gives us all that we need to do His work. "For it is God who works in you both to will and to do for His good pleasure." Philippians 2:13
I believe God wants to use each of us, and I believe it starts with a willing heart. From there He will use you to part the seas, big and small. :)