How can one stop thinking about the act of kissing that occurred, and is thinking about it after it happened considered impermissible (haram)?

The believer's heart must not be attached to anyone but Allah, and he must not contemplate disobeying Him; for faith necessitates the certainty that Allah is the Bestower of blessings, the All-Seeing, and it obliges shyness before Him and fear of His punishment. Prayer forbids indecency and evil, and reciting the Quran motivates one to act upon its teachings. The solution to the problem lies in sincere repentance, maintaining prayers, increasing good deeds, reciting the Quran with contemplation, filling one's time with what benefits, not surrendering to whispers (waswas), choosing righteous companionship, proactively seeking marriage with a religious spouse if possible, and staying away from the company of evildoers and places of temptation. Contemplating a forbidden act and indulging in such thoughts is forbidden; it is part of zina (unlawful sexual intercourse).