I thought we have to confess both Venial and mortal sins in the confessional.
Only mortal sins are obliged to be confessed. Apart from the rare case of perfect contrition at the moment of death, only sacramental absolution can remit the guilt of mortal sin.
The guilt (and temporal punishment) of venial sin can be remitted through reception of Holy Communion, devoutly praying an Act of Contrition, and the pious use of Holy Water amongt other ways.