Is there an expiation for the sin I committed, which did not reach the extent of major adultery, and does sincere repentance remove this sin and restore the pleasure of obedience?

The sin you committed does not necessitate a prescribed punishment (hadd). You must engage in sincere repentance, feel remorse, and resolve not to return to it. If you repent sincerely, Allah will erase your sin, and you will be as if you had not sinned, for the Prophet, peace and blessings be upon him, said: "The one who repents from a sin is like one who has no sin." No expiation (kaffarah) is required of you, but rather it is legislated for you to increase in good deeds and acts of obedience. Beware of abandoning prayer and neglecting obligatory duties, for this is a greater sin. Turn to Allah and repent to Him sincerely, safeguard the obligatory duties, abandon unlawful acts, forsake bad company, and supplicate to Allah to make you steadfast.