What is the Torah approach to stop yourself from feeling sexually aroused in the moment? Anything from the mussar experts?
אָמַר רַבִּי לֵוִי בַּר חָמָא, אָמַר רַבִּי שִׁמְעוֹן בֶּן לָקִישׁ: לְעוֹלָם יַרְגִּיז אָדָם יֵצֶר טוֹב עַל יֵצֶר הָרַע, שֶׁנֶּאֱמַר: ״רִגְזוּ וְאַל תֶּחֱטָאוּ״ אִם נִצְּחוֹ — מוּטָב, וְאִם לָאו — יַעֲסוֹק בַּתּוֹרָה, שֶׁנֶּאֱמַר: ״אִמְרוּ בִלְבַבְכֶם״. אִם נִצְּחוֹ — מוּטָב, וְאִם לָאו — יִקְרָא קְרִיאַת שְׁמַע, שֶׁנֶּאֱמַר: ״עַל מִשְׁכַּבְכֶם״. אִם נִצְּחוֹ — מוּטָב, וְאִם לָאו — יִזְכּוֹר לוֹ יוֹם הַמִּיתָה, שֶׁנֶּאֱמַר: ״וְדֹמּוּ סֶלָה״.
Rabbi Levi bar Ḥama said that Rabbi Shimon ben Lakish said: One should always incite his good inclination against his evil inclination, i.e., that one must constantly struggle so that his evil inclination does not lead him to transgression, as it is stated: "Tremble, and do not sin."
If one succeeds and subdues his evil inclination, excellent, but if he does not succeed in subduing it, he should study Torah, as alluded to in the verse: “Say to your heart.”
If he subdues his evil inclination, excellent; if not, he should recite Shema, which contains the acceptance of the yoke of God, and the concept of reward and punishment, as it is stated in the verse: “Upon your bed,” which alludes to Shema, where it says: “When you lie down.”
If he subdues his evil inclination, excellent; if not, he should remind himself of the day of death, whose silence is alluded to in the continuation of the verse: “And be still, Selah.”
The Talmud in Kiddushin 30b states:
תנא דבי ר' ישמעאל בני אם פגע בך מנוול זה משכהו לבית המדרש אם אבן הוא נימוח ואם ברזל הוא מתפוצץ שנאמר הלא כה דברי כאש נאם ה' וכפטיש יפוצץ סלע אם אבן הוא נימוח שנאמר הוי כל צמא לכו למים ואומר אבנים שחקו מים
The School of R. Ishmael taught: My son, if this repulsive [wretch] assail thee, lead him to the schoolhouse: if he is of stone, he will dissolve; if iron, he will shiver [into fragments], for it is said: Is not my word like as fire? saith the Lord,’ and like a hammer that breaketh the rock in pieces? If he is of stone, he will dissolve, for it is written: Ho, everyone that thirsteth, come ye to the waters; and it is said: The waters wear the stones.
Rashi explains מנוול as:
יצר הרע מתגרה בך
The Evil Inclination inciting you.
Similarly, Rambam in Hilchot Issurei Biah 22:21 writes:
וכן ינהוג להתרחק מן השחוק ומן השכרות ומדברי עגבים שאלו גורמין גדולים והם מעלות של עריות ולא ישב בלא אשה שמנהג זה גורם לטהרה יתירה גדולה מכל זאת אמרו יפנה עצמו ומחשבתו לדברי תורה וירחיב דעתו בחכמה שאין מחשבת עריות מתגברת אלא בלב פנוי מן החכמה ובחכמה הוא אומר אילת אהבים ויעלת חן דדיה ירווך בכל עת באהבתה תשגה תמיד
Similarly, a person should distance himself from levity, intoxication, and flirtation, for they are great precipitators and steps [leading] to forbidden relations.
A man should not live without a wife, for this practice leads to great purity. And [our Sages gave] even greater [advice], saying: "A person should always turn himself and his thoughts to the words of the Torah and expand his knowledge in wisdom, for the thoughts of forbidden relations grow strong solely in a heart which is empty of wisdom." And in [Solomon's words of] wisdom [Proverbs 5:19], it is written: "It is a beloved hind, arousing favor. Her breasts will satisfy you at all times. You shall be obsessed with her love."
The Baal Hatanya in Chapters 28-29 of Tanya instructs that if foreign thoughts are distracting someone during lay activities (not prayer or learning etc.) he should remove his mind entirely from those subjects (and instead control his mind to think about other things and especially Torah) and be joyous that he has the chance to engage in the mitzvah of לא תתורו וכו, not following his desires.
And if the thoughts are being aroused at times of prayer or learning he should increase his efforts in focusing knowing that the power of his prayer/learning are the cause for his yetzer (inclination) to work so hard to distract him!
For a translation of these chapters see https://www.chabad.org/1058817.
The Yetzer Hara unfortunately is a powerful thing and the challenge is how to combat it effectively. By way of introduction, it is worth noting what Shlomo Hamelech says in Koheles 10:4:
אִם־ר֤וּחַ הַמּוֹשֵׁל֙ תַּעֲלֶ֣ה עָלֶ֔יךָ מְקוֹמְךָ֖ אַל־תַּנַּ֑ח כִּ֣י מַרְפֵּ֔א יַנִּ֖יחַ חֲטָאִ֥ים גְּדוֹלִֽים׃
If the spirit of the ruler rises up against you, don’t give up your post; for slackening will make possible great sins.
in the book 'Pathways to Personal Growth'(Feldheim) p.121 it writes:
"The "ruler" is the yetzer hara. There are times when it "rises up against you", and all your progress seems to come to a halt and your mind becomes obessessed with alien thoughts. Your orders at such times are; "Do not abandon your place!" Despite everything. Do not abandon that which makes you into a human being - hang onto your status of "chooser" and "doer" (cf. the Targum)"
So the first point is to not let the moment slip. One should put defences in place to help fight against the yetzer hara. This will help deal with the situation to help effectively fight the urge which will build resilience if the urge returns in the future.
Indeed the Sefer HaChinuch 188 writes how no person is impervious to the Yetzer hara and thus we need 'fences' in place to spare us from falling.
Approach 1 - Torah
The Gemara in Kiddushin 30b writes:
כך הקב"ה אמר להם לישראל בני בראתי יצר הרע ובראתי לו תורה תבלין ואם אתם עוסקים בתורה אין אתם נמסרים בידו שנאמר (בראשית ד, ז) הלא אם תטיב שאת
So too the Holy One, Blessed be He, said to Israel: My children, I created an evil inclination, which is the wound, and I created Torah as its antidote. If you are engaged in Torah study you will not be given over into the hand of the evil inclination, as it is stated: “If you do well, shall it not be lifted up?” (Genesis 4:7). i.e. One who engages in Torah study lifts himself above the evil inclination. (Sefaria translation)
Approach 2 - Prayer
The Chazon Ish (Kovetz Igros Chazon Ish, cheilek alef, letter #2 - apologies I can't find it online) writes:
""Prayer is a staff of strength in the hand of every man." Prayer has the power to keep you at your present level in spirituality and in the end it will enable you to carry on with your ambitions."
Approach 3 - Treat the Yetzer Hara with contempt
A final method is to train our minds to think of the Yetzer Hara with a sense of disdain. Yosef HaTzaddik is a famous example of someone who was tempted but succeeded against the lure of his Yetzer Hara. In Bava Basra 109b we're told that Yosef was "פטפט ביצרו" - that "he battled with his Yetzer". Similarly the Gemara in Gittin 57a this same expression is employed to describe a man who showed a great deal of control - "שפטפט ביצרו יותר מיוסף". Rashi over there explains this to mean that the way he was able to overcome the experience was "זלזל ביצרו", i.e. he treated it with disdain.
In Pathways to Personal Growth (pp.146-47) it writes effectively:
"The first step to overcoming the yeitzer is to treat it with disdain, to demote it from its position of command and to put it in its proper place: "Trua, I have a desire for such and such a sin. But this is by no means my entire world or my reason for living! It is merely a marginal craving of little value that can be ignored with little difficulty. Considering the harm it will cause it would be insane to give in." In this manner Yosef deflated his Yeitzer". One must put his desires in perspective. The urge to commit a transgression seems overwhelming only if a person attributes too much significance to his desires. A little reflection is all that is necessary to put the yeitzer in its place and to realize that the fantasy image it has painted is in conflict with the reality. Then it will simply melt away and evaporate."
The Aley Shor (vol 1 page 41) offers that there is nothing greater to stave off the desires that immersing oneself in learning gemorah with rashi and tosefos with hasmadah (diligence). I wish you much success in succeeding in this matter. If your hebrew is good, read his whole piece on this topic from pages 39-41. It helps even if one isn't so successful with this.