Yeah snowball, all forms of sɛҳuąƖ activity outside of marriage are mortally sinful except for nocturnal emissions. Even the thought alone is mortal if sufficiently entertained... even if only for a second!
With that in mind, my only other suggestion for the OP outside some of the very good meditations and prayers that have been suggested here is this: use the Sign of the Cross whenever an impure thought or memory enters into your mind. It's vital to catch them as soon as we are aware of them and for young men this can be many, many times in one day. The Sign of the Cross is quick and easy to do and will be able to match the pace of any temptation that bears down on you. Let the Cross do the work and any other pious image you can produce in your mind's eye.
.
Ever since Vat.II there have been priests teaching children lies by telling them they can fool around with like heavy petting etc., and it's not sinful. That only leads to moral corruption and comes to no good.
.
Our culture (if you can call it that with legalized abortion and pornography) fairly glorifies impurity. I have known many contemporaries, especially Europeans but the number in America has been growing for 30 years, who firmly believe that to entirely exclude impure thoughts from one's mind one could not possibly be human. They truly believe that it is inherently human to habitually indulge in prurient interest. Furthermore to even attempt to resist such thoughts is equivalent to cheating yourself out of one of life's ESSENTIAL pleasures.
.
Nothing could be further from the truth.
.
Take the news stories swirling around the Harvey Weinstein scandal like maggots on a corpse. I heard a Hollywood movie office representative responding to a man who was threatening to report a case of producer sɛҳuąƖ harassment, by telling him "If you do that, you could ruin an entire industry."
Get it? By your REPORTING sɛҳuąƖ impropriety YOU become the problem, and the reason behind this thinking is, that movie producers and power brokers do this all the time and that's what makes Hollywood special.
.
Get it?.
When Hugh Hefner was about to die, he was asked by a journalist if he thought he had led a full life, and why he had not settled on just one woman. He had two answers, since it was two questions. He said that he could not imagine any life sweeter than this one, and that he had never met his soul mate. The message here is that there is far more pleasure to be found in adultery and fornication and running a pornography empire than there ever could be by living a faithful life with one spouse. Which is another big lie of our modern world.
.
I knew a man who loved to stare at scantily clad women, and when I said anything about it, his response was,
"And they wonder why there's rape."
.
I knew another man who found himself unable to avoid alley encounters with prostitutes, which made no sense to me, so one time when he was telling me about how a particular scene unfolded he said he was talking to her and she started walking around the dumpster into the alley and he had to follow her or else he wouldn't be able to understand what she was saying since her voice was soft and she was saying things he wanted to hear, so he followed her so he could understand her words, and one thing led to another until before you know it... I interrupted him and said,
"You have to realize that at some point along the way you made a decision to keep going. I have no question but that if I were to find myself in the exact same situation, I would never have gone through with what you went through with. When she starts leading you around like that you simply do not follow her. Hey, you don't know if she had a pimp back there waiting for you to walk into a trap and get mugged! Besides, what are you doing talking to girls you know are prostitutes in the first place?" He assured me they have a rough life and he likes to help them pay their bills. UUGGGHH.
.
With sins of impurity it's always the same. It starts with a thought. When you give in to the temptation to think about an impure action, you have already given your consent, and continuing to think about it quickly becomes a mortal sin.
Any priest who does not make that fact perfectly clear is actually committing a very grievous mortal sin of his own, because of all the listeners who will believe him and then go out living impure lives because of what a priest told them. Such bad advice teaches weak people to play with fire and the more they play with it the sooner they'll get burned, it's not if but when.
.
His prison attendants tried to tempt St. Thomas Aquinas with a prostitute and he grabbed a heavy club threatening to beat her up and she ran away screaming. St. Francis of Assisi rolled around in thorny bushes to quell his body's urges that he wanted to suppress. The Fatima children wore rough cord ropes tightly about their waist, and they never mentioned having to fight against impure thoughts, so they were probably too young for that in the first place.
.
Blessed Jacinta Marto had to endure a severely painful operation for purulent pleurisy with fistula, osteitis of the 7th and 8th ribs of the same side. She went under the knife to have 2 ribs removed, without effective anesthesia. Going into that operation she knew the level of pain she would suffer but she was not afraid of that. There was only one concern she had, one fear, and that was her fear that while letting the hospital workers expose her body for preparation, her nudity might tempt one of the male workers to have thoughts of impurity. That is the only time she is on record of ever mentioning thoughts of impurity, and it wasn't even about HER thoughts. She specifically offered all her sufferings for the conversion of sinners to save them from hell, and for the Holy Father.
.
Now THAT is a model of heroic virtue. Anyone who can think of Blessed Jacinta in prep for surgery and still have an impure thought is a sorry sight indeed.
.