How can one strengthen faith and sincerity with Allah after a relapse and almost an addiction to masturbation?

Your commission of sin may be what prevents you from feeling the sweetness of faith, for sins distance one from Allah. To strengthen your faith, repent sincerely, abandon sins immediately, and strive against your soul, believing that Allah will help you if He sees sincerity and devotion from you, in accordance with His saying, "And those who strive for Us – We will surely guide them to Our ways. And indeed, Allah is with the doers of good." Distance yourself from what excites your desires, increase in fasting, and adhere to the remembrance of Allah and supplication. Reflect on Allah's names and attributes, and on death and what comes after it, for that softens the heart. Accompany the righteous, attend gatherings of remembrance and knowledge, perform the obligatory prayers, and increase in supererogatory acts; for by them, a servant draws near to his Lord until He loves him.