"How can I be sure I am doing what He wants me to do?"
Anything we do, God Himself can do better, so what, after all, does God want from us?
And what does the Lord require of you? To act justly and love mercy and to walk humbly with your God.--Micah 6:8
Yes, yes, I know that. But what to DO?
God gives us stuff to do not because He needs us to get it done for Him, but because He wants us to seek Him in it.
Seek Him first, last, and always...then do what seems right until we can't do it any more.
Don't concentrate on the result.
As the heavens are higher than the earth, so are my ways higher than your ways and my thoughts than your thoughts.--Isaiah 55:9
Do what God gives us to do because He gave it.
He manages the result.
I planted the seed, Apollos watered it, but God made it grow.--1Corinthians 3:61
We cultivate devotion to God. He brings in the harvest.
Beethoven was nearly deaf when he composed his ninth symphony. He never heard it, but he wrote it, and conducted it, with such genius and fervor that almost everyone recognizes its Ode to Joy:
God asks us, too, to play the notes even when we can't hear the music.
My heart is steadfast, O God, my heart is steadfast. I will sing and make music. Awake, my soul. Awake, harp and lyre. I will awaken the dawn.--Psalm 57:71
Play on, and our love for God becomes our true song.