But Moses pleaded with the LORD, “O Lord, I’m not very good with words. I never have been, and I’m not now, even though you have spoken to me. I get tongue-tied, and my words get tangled.” Then the LORD asked Moses, “Who makes a person’s mouth? Who decides whether people speak or do not speak, hear or do not hear, see or do not see? Is it not I, the LORD? (Exodus 4:10, 11 NLT)
If you feel adequate in yourself to do the Lord's work then you likely have a problem with pride. If you refuse to do the Lord's work because you feel inadequate in yourself...you are being disobedient. If you yield yourself to the Lord you'll find that He is more than adequate to do His work through you!