What should a religiously committed young man do who had hoped to marry a particular girl, but was shocked by her engagement to someone else, which led to a bad psychological state that pushed him to watch pornography, and he wants to get out of this cycle?

A believer should not confine their goals to worldly pleasures. Rather, their greatest concern should be to attain the pleasure of their Lord and achieve His Paradise. Allah is All-Wise; He places things in their proper places and gives each person what is befitting for them. His choice for a servant is better than the servant's choice for themselves. Trying to forget by escaping to watch pornography is fleeing from the scorching heat to the fire. Therefore, one must repent and flee to obedience to Allah and have good thoughts of Him, occupy oneself with what benefits in this world and the Hereafter, supplicate to Allah to remove the attachment, and strive against the self by avoiding thinking about her.