What can I do to get rid of old memories?

A believer should occupy himself with acts of obedience, increasing in remembrance [dhikr] and recitation of the Qur’an, and seek refuge with Allah from the accursed Shaytan, and strive to keep company with the righteous. If a Muslim recalls a sin he has repented from and his soul does not find pleasure in it, then that does not harm him. However, if he recalls it and finds pleasure in it, then that is a new sin that necessitates repentance. The Prophet, peace and blessings be upon him, used to sit after the Fajr prayer and converse with his Companions about matters of the time of Jahiliyyah, and they would laugh while he, peace and blessings be upon him, would smile.