These sorts of sins can seem intractable because they form powerful habits that reach into several different layers of the person (physical, spiritual, affective, etc.) and because they are often smokescreens for other problems.
You fool! You have really done what you did not want to do! God has left you with the pleasure, because the pleasure also was His will: but you have neglected the happiness He wanted to give you along with the pleasure, or perhaps the greater happiness He intended for you without the pleasure and beyond it and above it!
You have eaten the rind and thrown away the orange. You have kept the paper that was nothing but a wrapping and you have thrown away the case and the ring and the diamond.
--Thomas Merton, New Seeds of Contemplation, 201-202.
Should there be a desire for repentance and amendment of life, one of the questions someone might ask is how much they are willing to suffer in order not to displease God. In this spirit I love this story from the Desert Fathers, not only because it is a tale of utterly committed ascesis, but also because it shows how genuine love between Christians can help defeat the "demon of fornication."
Abba Phocas also said, 'When he came to Scetis, Abba James was strongly attacked by the demon of fornication. As the warfare pressed harder, he came to see me and told me about it, saying to me, "Tomorrow, I am going to such and such a cave but I entreat you for the Lord's sake, do not speak of it to anyone, not even my father. But count forty days and when they are fulfilled do me the kindness of coming and bringing me holy communion. If you find me dead, bury me, but if you find me still alive, give me holy communion."
Having heard this, when the forty days were fulfilled, I took holy communion and a whole loaf with a little wine and went to find him. As I was drawing near to the cave I smelt a very bad smell which came from its mouth. I said to myself, "The blessed one is at rest." When I got close to him, I found him half dead. When he saw me he moved his right hand a little, as much as he could, asking me for the holy communion with his hand. I said to him, "I have it." He wanted to open his mouth but it was fast shut. Not knowing what to do, I went out into the desert and found a piece of wood and with much difficulty, I opened his mouth a little. I poured in a little of the body and the precious blood, as much as he could take of them.
Through his participation in the holy communion he drew strength. A little after, soaking some crumbs of ordinary bread I offered them to him and after a time, some more, as much as he could take. So, by the grace of God, he came back with me a day later and walked as far as his own cell, delivered, by the help of God, from the harmful passion of fornication.'
This is from this edition of the Sayings, which is my favorite in English.