"Take heart, child, your sins are forgiven."
The only thing more important than prayer is to become silent so as to hear the Word of God - the voice of God from the lips of Jesus.
When you pray, "God, be merciful to me, a sinner," what does this God say back to you? His reply is found in Matthew 9:2: "Take heart, child, your sins are forgiven." Understand that you are the one to whom these words are addressed.
"Take heart." This means a cheerful, confident heart, full of courage. For when you hear and believe that your sins are forgiven, then nothing big or small can take away your joy and strength. Your heart becomes a place in which a bright morning sun is always rising. Yes, the trials still come, but the verdict is in and it is final: Your sins are forgiven.
"Child." Use the word "child" to remember your Baptism. And will not the one who calls you "child" always love and care for you in every way? Other times, in place of the word "child," use your full first name. This can be a very powerful experience! Try it: "Take heart, [name], your sins are forgiven."
So now you have your seven words: "God, be merciful to me, a sinner." And God has His: "Take heart, child, your sins are forgiven."
Enter into conversation with Him often.