What should we do when our spiritual life isn't what it should be?
I think it's a good place to be in this life, to be aware that we're not what we ought to be. The Apostle Paul set a clear example for us in Philippians 3:12 when he said, "Not that I have already obtained this or am already perfect, but I press on to make it my own, because Christ Jesus has made me his own."
So the thing to keep in mind when I sense my inadequacy as a Christian is that my acceptance with the Father does not depend ultimately on my adequacy, because Christ has made me his own. He has reached down and—by his cross, by election, by his calling, and by his regeneration—made me his own.
I want to keep my focus on him and pray, "O God, incline my heart to you. Keep me, use me," and then grow in grace by looking to the one who has made me his own.