Isaiah stood before the Lord feeling ashamed, unworthy and embarrassed.
Then within moments he was boldly declaring to the Lord “here am I; send me”.
Isaiah didn’t give himself a great pep talk that brought on the change. Instead the change came about when he was cleansed of his iniquity-when his sin was purged. 
What an illustration for us.
When we walk in darkness we aren’t capable of doing kingdom work. We aren’t qualified. Our feelings of unworthiness are valid. 
But when we accept the blood of Jesus as an atonement for our sin we’re immediately transformed. Christ makes us worthy. His blood qualifies us instantly. 
So Christian, put away the feelings of inadequacy, unworthiness and doubt. Just come to the Lord saying “here am I; send me.” and then follow where He leads.