Writing on Psalm 51, Luther says:

I have learned from my own experience that praying is often the most difficult thing to do.  I don't hold myself up as a master of prayer.  In fact, I admit that I have often said these words coldly: "God, have mercy on me."  I prayed that way because I was worried about my own unworthiness.  Yet ultimately the Holy Spirit convinced me, "No matter how you feel, you must pray!"  God wants us to pray, and he wants to hear our prayers--not because we are worthy, but because he is merciful.

(from Faith Alone: A Daily Devotional / LW 12:314)

"...not because we are worthy, but because he is merciful"  Beautiful reminder!