I find the following two prayers to be helpful for overcoming sexual sin and temptation. I got both from a Christopher West book I read.
Lord, please help me. Give me the grace to trust you with my whole self--all that I am, body & soul. I give up my hopes & fears, my achievements & failings, my strengths & weaknesses, my sins, my longings, my desires--especially, right now, my sexual sins, longings, & desires. I lay them all at your feet. Help me to be the woman (man) you've created me to be. Renew my mind that I might see the great gift of sex & marriage as you've created them to be. I know I cannot live your will on my own, but I trust you to make up what I am lacking. Amen.
Lord, you have created me in your image & likeness as a woman (man). Help me to accept & receive my sexuality as a gift from you. You have inscribed in my very being, in my sexuality, the call to love as you love, in sincere self-giving, & you have made the "one flesh" union of man & woman in marriage a sign of your own life & love in the world. Grant me the grace always to resist the many lies that continually assail the truth & meaning of this great gift of sexuality. Grant me purity of heart so that I might see the image of your glory in the beauty of others, & one day see you face to face. Amen.