How can one permanently get rid of masturbation in light of repeated relapses and the fear of falling into adultery?

For repentance and steadfastness, you should associate with righteous people, engage in remembrance (dhikr (definition)), supplication (du'a), frequent fasting, and stay away from temptations. If you return to sin, repent again, for Allah is Forgiving and Merciful, and one who repents from a sin is like one who has no sin. Continue to repent whenever you sin until Allah blesses you with sincere repentance (tawbah (definition) nasuh). Your repentance is valid and accepted as long as it fulfills its conditions, such as regret and a firm resolve not to return to the sin. Beware of despairing of Allah's mercy.