How can one overcome the moment of weakness that leads to watching pornography and practicing masturbation, despite repeated repentance and attempts to abstain from them, and is there a solution to change this situation?

Your feeling of pain and prick of conscience is an indication that your heart is alive and well, and a believer is he whom his good deed delights and his evil deed grieves. You must strive sincerely, repent frequently, accompany righteous people, engage in da'wah and scholarly activities, and supplicate often. Marriage soon is the most effective cure for the ailment of lust; otherwise, you must fast. Reflect often on the names and attributes of Allah, and on death and what comes after it.