How can one overcome the inability to forget their first beloved, especially after her engagement, and what is the way to move past the impact of that on academic and psychological levels?

Among the fruits of loving Allah and His Messenger is obeying their commands and abstaining from their prohibitions. This includes the prohibition of illicit intimacy (mukhādanah), unlawful seclusion (khalwah), and sleeping in the same bed. The point is not merely the absence of thought about indecency, but rather the bad deed itself and following the footsteps of Satan, which lead to it. One should offer sincere repentance (tawbah (definition) nasuh), seek Allah's help to forget this relationship, and entrust the matter of marriage to Him, asking Him for a righteous wife.