Why is it that Thou hast sent me? - Exodus 5:22

BEFORE God can use us, He must bring us to an end of ourselves. When Paul was summoned to the greatest epistles and labors of his life, his strength was drained to utter weakness, and he despaired even of life. So in the case of Moses and Israel.
 
Moses, for forty years, had been undergoing the emptying process; but perhaps when God called him to this great enterprise, there may have been a slight revival of confidence in himself, in his mission, his miracles, the eloquence of Aaron's speech. So in the rebuff he received from Pharaoh, in the bitter remonstrances of the elders of his people, in the sad consciousness that his efforts had aggravated their condition, the lesson was still further taught him - that of himself he could do absolutely nothing.
 
Israel also had begun to hope something from his mission. Through the brickfields the story ran of his early years, his uncompromising speech to Pharaoh, of his miracles; and the wretched slaves cherished faith in him and Aaron as their heaven-sent deliverers. They had, however, to learn that all such hopes were vain, and to see that the brothers, at the best, were as weak as themselves. Then the way was prepared to lean only on God.
 
Ourselves. - By repeated failures all along our life-course God is teaching us the same lesson. We fail to justify and then to sanctify ourselves. Our efforts to serve and please Him only end in increasing perplexity. The tale of bricks is doubled; the burdens augment; the strength of our purpose is broken; we are utterly discouraged; and then, when the soul is utterly desolate, the heavenly Bridegroom draws near and says, "I will do all; I am Alpha and Omega; I am thy salvation."