"But the tax collector, standing far off, would not even lift up his eyes to heaven, but beat his breast, saying, 'God, be merciful to me, a sinner!'" (Luke 18:13 ESV)
"Prayer must not be based on or depend on your personal worthiness or the quality of the prayer itself; rather, it must be based on the unchanging truth of God’s promise. If the prayer is based on itself or on anything else besides God’s promise, then it’s a false prayer that deceives you—even if your heart is breaking with intense devotion and you are weeping drops of blood.
"We pray because we are unworthy to pray. Our prayers are heard precisely because we believe that we are unworthy. We become worthy to pray when we risk everything on God’s faithfulness alone. So go ahead and feel unworthy. But know in your heart that it’s a thousand times more important to honor God’s truthfulness. Yes, everything depends on this alone. Don’t turn his faithful promise into a lie by your doubts. For your worthiness doesn’t help you, and neither does your unworthiness hinder you. A lack of faith is what condemns you, but confidence in God is what makes you worthy."
~ Martin Luther, from Faith Alone: A Daily Devotional by Martin Luther.