*Venishmarta mikol davar ra, mikan shelo yeharhar adam bayom veyavo lidei tumah balailah. Why does the Torah tell us the prohibition of being motzei zera levatala in a roundabout way, venishmarta etc.? why doesn't it say it in a direct fashion, exactly what is assur?
The answer is that a person's primary responsibility is to make safeguards and fences for himself so that he will not come to being mz"l. Guard your eyes from seeing what they shouldn't, stay away from things that trigger you, don't put yourself into a matzav where you will have a nisayon. that is your job, to keep away from things that can cause you to be overcome with taivah. venishmarta, and then you won't have the tumah.
Shemen Rosh, vol. 6, Parshas Vayechi
*Vayevarchem bayom hahu leimor, Yakov gave the shevatim a brachah that they should always see only the current day, and not worry and fear for tomorrow.
Reb Moshe Kobriner, Toras Avos
*Chaza"l say that the Yetzer Hara has no power over someone who guards his eyes (Yalkut Shimoni, Vayechi). And conversely, Chaza"l also say that one who does not guard his eyes is destined to fall into sin (Yalkut Shimoni, Bamidbar).
Many of us on this site can attest the truth of the words of chazal. Many of us who have worked on this, even for a small amount of time, know that within a few days the Y"H simply slips out of site. The main trick is maintaining this for life.