Is the repentance of an addict from a certain sin considered sincere repentance (tawbah nasuh) if he resolves not to return to it, but deep down he knows he will return, and how can he thank God for His blessings while he disobeys and repents?

If a servant repents with sincere repentance, fulfilling its conditions—namely, abandoning the sin, regretting it, and resolving not to return to it—then Allah accepts his repentance. Even if he returns to the sin and then repents with sincere repentance, his repentance will be accepted. Allah knows the sincerity of repentance, and whoever does not resolve not to return has not fulfilled the conditions of repentance. Among the things that help one remain steadfast in repentance are cutting ties with the causes of sin and replacing them with what aids in obedience. As for gratitude, it is expressed by using blessings in obedience to Allah and not in disobedience to Him, and by praising and extolling Allah with the tongue. If a grateful person falls into sin, he must hasten to repent from it.