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.

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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.
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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.
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Nothing could be further from the truth.
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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. 
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Get it?
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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.
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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." 
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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.
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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.
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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. 
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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.
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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.
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One of the most powerful prayers is said at the beginning of all prayers in the Eastern Catholic Church.
It is addressed to the All-Holy Spirit, and it begs for purity (purity of intention and attentiveness) and the cleansing of all impurity.

"Glory to Thee, Our God. Glory to Thee.

O Heavenly King, O Comforter, O Spirit of Truth,
Who art everywhere present and fillest all things,
Treasury of blessings, and Giver of Life,
Come and dwell in us; Cleanse us of all impurity;
And save our souls, O Good One.

Holy God, Holy Mighty, Holy Immortal One, have mercy on us (3x)"

I pray this prayer whenever I encounter any temptation. It provides immediate help.

The Catechism of the Catholic Church says that:
Has this teaching been inserted in previous catechisms? And, by saying "that lessen, if not even reduce to a minimum, moral culpability" does it mean that under some circuмstances, it might NOT EVEN BE A SIN at all? I'm confused now. :o :o

"Mission complete".

Some sins, by their very nature, are mortal; this is one of them. 

You should keep minding that pesky sense, and keep asking questions.

The three hail Mary's prayed in the morning and before bedtime along with the daily rosary are powerful weapons and absolutely must be employed every single day. To this must be added fasting.

We commit sins of impurity, much the same like all sins, i.e. in order to satisfy our selfish desires. Overcome our selfishness and we overcome committing sins of impurity. For most (all?) of us, being selfish is second nature. When we constantly strive to make the effort to habitually give up things for the souls in purgatory, or the glory of God, or for whatever noble reason, we train ourselves in self sacrifice, weaning us out of habitual selfishness with the goal of making it second nature for us to be completely unselfish through self sacrifice.

Fasting is the best and simplest way I know of to get in the habit of self sacrifice. Simply begin by skipping a desert once a day, or having three pancakes instead of four type of thing. I assure you that in only a short time you will know how much fasting you need, but getting in the habit of routinely denying yourself little pleasures, will in only a very short time be an invaluable aid in denying yourself sinful pleasures.

   

Yeah, this subjectivist stuff is a slippery slope.  While there's undoubtedly a subjective component to any sin, understood even in Traditional moral theology, emphasis on the subjective aspect quickly bleeds over into an erosion of the objective side of things.

That's exactly what Francis Bergoglio did with regard to the divorcee fornication issue.

While it is theoretically possible that in some cases culpability can be greatly reduced due to such subjective considerations (addictions, habits, and the like) ... that determination can only be made by God.  So, for instance, if I have never injected heroin before, and I decide to take it, knowing full well that it's a mortal sin, certainly that's a different degree of culpability than an individual isolated failure of the serious addict who almost can't help himself (one a cold, calculating decision ... the other a fall in a desperate state of weakness).  Nevertheless, the presumption must always be that if an objectively grave act has been committed, it is to be treated like a mortal sin and confessed as such.  With regard to the divorce problem, Bergoglio basically decided that an individual and his/her confessor could "discern" the degree of this culpability.  That's utter hogwash.  Only God can do that.  Only God can read the internal forum so as to make that determination.  So the saying goes that De internis ecclesia non judicat. ("The Church makes no judgments regarding the internal forum.")  Similarly here with the sin of self-abuse.  It's possible, even very likely, that there's diminution of moral culpability in the eyes of God in many/most cases, but the act must be treated by us as objectively a grave sin.

Otherwise, what are the consequences?  Hmmm, "I don't think I was fully culpable of this sin."  Next step is that I don't consider it a mortal sin anymore in the practical order.  Next step after that is that I don't even try very hard not to commit the sin ... since in my judgment it's not mortal.  And then, ironically, if it wasn't a mortal sin then, it most certainly is now (when there's no effort to curb it).  Consequently, this subjective consideration bleeds into the objective moral order and essentially destroys it, effectively eliminating the notion of grave sin.