One of the standard conditions for receiving a plenary indulgence is sacramental confession. What if I wish to receive a plenary indulgence for some act of devotion, but am not aware of any unabsolved mortal sins on my soul? What do I confess?
Confess your venial sins. The Church recommends the confession of venial sins (cf. Code of Canon Law, 988.2) as it “helps us form our conscience, fight against evil tendencies, let ourselves be healed by Christ and progress in the life of the Spirit. By receiving more frequently through this sacrament the gift of the Father’s mercy, we are spurred to be merciful as he is merciful” (Catechism, 1458).