What is the ruling on repeatedly supplicating for marriage to an engaged person, is there sin in that, and what is the way to forget him?

There is no harm in supplicating for a specific person for marriage, although a general supplication is better due to not knowing where the good lies. To treat infatuation and attachment to a non-mahram, one must lower the gaze, avoid seclusion and idleness, increase in the remembrance of Allah, seek refuge from Satan, and associate with righteous women.