How can one practically repent from committing a sin with a girl and feeling regret for it, especially when there is no feeling of guilt and considering that the questioner is a hafiz of the Qur'an and married, but not sexually comfortable with his wife?

Adultery is a grave and heinous sin. Its ugliness is compounded when committed by a married person who has memorized the Qur’an. The adulterer must immediately repent and feel remorse before it is too late. He should recite the Qur’an and ponder its verses to learn a lesson. The absence of admonition in his heart led him to commit adultery, so he must fear Allah and safeguard His Book. If one wife is insufficient to fulfill desires, Allah has permitted marriage to more than one woman. Fasting, righteous companionship, and supplication can also be helpful in staying away from this vice.