How can one sincerely repent from watching pornography and masturbating when repeatedly returning to the sin after every repentance?

Do not despair of God's mercy, for the door of repentance is open no matter how many times a sin is repeated or how great it is. The remedy for sins is sincere repentance. If you repent sincerely, fulfilling its conditions (abstaining, resolving not to return to the sin, regretting it), then it is a sound, accepted repentance that erases the effect of the sin. If you return to disobedience, you must repent again, and continue to do so.

Among what helps you to remain steadfast in repentance are: contemplating the ugliness of the sin, its danger, and its punishments (including the sealing of the heart), striving against the self, keeping company with the righteous, diligent supplication, and occupying oneself with beneficial words and actions. It is recommended to read the book "Ad-Daa' wa Ad-Dawaa'" (The Disease and the Cure) by Ibn al-Qayyim.