Exodus 4:14 So the anger of the Lord was kindled against Moses, and He said: “Is not Aaron the Levite your brother? I know that he can speak well. And look, he is also coming out to meet you. When he sees you, he will be glad in his heart.
This is not just about not believing that you can do anything through God who strengthens you, this is about trusting and believing that God will do what He has promised!
When God gives you the idea to go out and do things for Him, He is going to make sure that you can do things even when you think you cannot.
Moses thought he could never be a leader because of his problems with his speech; but God had already made the plan, he just had to have faith!