This psalm is especially written by David, and outside the number of 150 psalms, you are always fighting against Goliath.
He was small among my brothers and the youngest in my father’s house, a shepherd of my father’s sheep. My hands have created an organ, and my fingers have formed a psalter. And who will tell my Lord? The Lord Himself will hear. He himself sent his angel, and took me from my father’s sheep, and anointed me with his anointing oil. My brothers are good and great, and the Lord has not been pleased with them. I died to meet the foreigner, and their idols cursed me. And I, plucking the sword from him, beheaded him, and took away the reproach from the children of Israel.
According to the 20th kathisma, the Trisagion of Our Father: Also troparia, tone 8:
Fulfillment of all good things / You are, my Christ, / Fill my soul with joy and gladness, / And save me, for I am the One who is Most Merciful.
Glory: Even though I have sinned, O Christ my Savior, before You, / do I know no other God than You, / I dare in Your goodness, / accept me who turns, O Compassionate Father / and Son O only begotten and holy soul, / like the prodigal son, save me.
And now: We do not know another refuge and warm Representation, / for You, / for we have the boldness of one born of You, / Lady, help and save me, Your servant.
Lord, have mercy (40) and prayer:
Lord Jesus Christ my God, have mercy on me, a sinner, and forgive Thy unworthy servant for the sins I have committed throughout my entire life, and even to this day, and yet man has sinned, my voluntary and involuntary sins, in deed and word, in mind and thought, even from co-admiration and inattention, and many of my laziness and negligence. If I have sworn in Your name, or sworn falsely, or blasphemed a thought, or reproached someone, or slandered, or been sad, or in wicked anger, or stolen, or fornication, or lying, or secretly poisoned, or a friend who came to me and despised him, or a brother who was insulted and bitter, or I stand in prayer and psalmody, my evil mind is on the evil one walked around, or enjoyed himself more than anything, or laughed madly, or spoke blasphemously, or became vain, or were proud, or saw the kindness of vain things and was deceived by it, or the ridiculous one mocked me.
Even if I was careless about my prayer, or did not keep the commandments of my spiritual father, or idle talk, or did other evil things, this is all that was done in these deeds, and those below. I remember. Have mercy, Lord, and forgive us all, so that I may sleep and rest in peace, singing, blessing, and glorifying Thee, with Thy Beginning Father and with Thy Most Holy, and Good, and Life-Giving One By the Spirit, now and forever and to the ages of ages. Amen.