What should one do who repents of a sin and then returns to it, having covenanted with God and sworn by His Book to abandon it, then breaks that oath, while feeling remorseful for his action and detesting disobedience to God?

Abandoning sins is a religious obligation, and the pledge and oath confirm the cessation [of sin]. One must fulfill their pledge and oath, especially if it pertains to abandoning a sin. You must repent from the sin and the breaking of the pledge, along with an expiation for each breach of the oath. This expiation is feeding ten poor people, or clothing them, or freeing a slave. If you cannot find [any of these], then it is fasting for three days. Do not despair of Allah's mercy and persevere in sincere repentance, for Allah loves those who repent, and the one who repents from sin is like one who has no sin.