The Midrash (rabbinic teaching) tells us that as long as a person is always learning Torah and teaching Torah, their heart is open and available, and their mind is open and available to learn more Torah. The Torah comes into a person's brain, soul, and heart. The evil inclination cannot attack, or at least has less of a chance of attacking. Another person cannot remove the Torah from that individual. For a person who is constantly involved in Torah study, whether in learning or teaching, the Torah will stick with them.
Now the Midrash continues and gives an anecdote. This is a parable, an anecdote about a king of human flesh. He was walking in the desert and found himself a very large palace with many, many large rooms in it. He moved in and went to live there. This anecdote is the same story about the yetzer hara (evil inclination). The Midrash tells us that the way the yetzer hara operates is that it looks for openings. It looks for room in one's mind. As soon as it finds itself there, unfortunately, it's very hard to remove it.
Rabbeinu Yonah explains the very famous Mishnah in Masechet Avot (Tractate Avot), which tells us that a person should not spend too much time speaking with women, even with one's own wife. He explains that the nature of the human mind is that the mind and the heart cannot be constantly involved in both realms: realms of learning Torah and realms of women. The mind and the heart cannot coexist. A person has to choose. Do they want to fill their brain with the wisdom of God and His Torah, or with promiscuity and inappropriate images and thoughts?
The Chafetz Chaim explains that Hashem has a set amount of Torah for each person to learn, and it is a huge amount. Every single one of us has the option and the opportunity to grow into that, learn that, and gain from all of that. However, he says that as long as a person has, as he explains it, an el zar, a foreign god, which, as he explains, means improper thoughts, immorality, promiscuity, and basically, women, then, unfortunately, the person will not be able to achieve that goal and that potential Hashem has set for them.
Then the Netziv writes that in the same way that Torah protects a person from illicit relationships and anything inappropriate, so too, unfortunately, when a person is involved in inappropriate thoughts or views things that are inappropriate, that also chases away their ability to learn Torah and to merit learning Torah. Unfortunately, he also writes that the more inappropriate thoughts and images a person looks at, the coarser their mind becomes. Rabbi Yonatan Eibeschutz tells us that a person's level of concentration is lowered tremendously when they are constantly thinking about, or have the opportunity to view, things that are inappropriate. It's not only in a person's learning, but also in a person's Torah study, teaching, prayer, and just concentrating on anything else that a person has to do, including work and anything similar to it.
There was once a man who came and asked a Gadol HaDor, one of the great Torah giants, for a segula, an advice, and a good luck for improving his memory. I guess the individual was having a hard time remembering anything, including the Torah study that he learned. The rabbi answered, “It's an obvious pasuk (verse) in the parasha of Tzitzit (fringes) that we see in the Keriat Shema (recitation of the Shema). It says, ‘A person should not follow their hearts and their eyes.’” As we know, following after one's eyes there means gazing at things that are inappropriate. The rabbi ends by saying that when a person looks at things that are inappropriate, unfortunately, that destroys one's memory.
Again, if a person wants to learn Torah, remember the Torah that they learn, and be illuminated, as we say in our prayer, Veha'er eineinu beToratecha, Hashem: please, illuminate our eyes in your Torah. Help us understand it. Help us remember it. We have to realize that guarding our eyes will help us with that. May Hashem help us and give us strength to guard our eyes so that we will learn, understand, and remember His holy Torah and apply it to our lives. Amen, ve'amen (Amen and amen).