How can one achieve devotion (khushu') in prayer (salat) and recitation of the Qur'an, and does the lack of complete devotion affect the acceptance of acts of worship, even while being diligent in performing obligatory duties and avoiding what displeases Allah?

Congratulations on maintaining your prayers and striving to be present in them. We recommend that you continue to struggle against your lower self to achieve presence of heart and to ponder the meanings of the Qur'an and the adhkar (remembrances). Humility (khushu') in prayer is highly encouraged and expiates sins. Some scholars have deemed it obligatory, but according to the Maliki school, prayer is not invalidated by its absence. The majority of scholars do not consider it obligatory, and both views agree on the validity of the prayer. You are rewarded for reciting the Qur'an even if you do not ponder its meanings, but you are encouraged to do so, as Allah says: "That they may ponder over its verses" (38:29) and "Do they not then ponder over the Qur'an?" (47:24).