The remission of venial sins occurs, when one is in a state of grace to begin with, during the reception of the Holy Eucharist. Of mortal sins we know that we must receive the sacrament on Confession, lest a sacrilege is committed and we bring down condemnation upon our heads. See St Paul's letter to the Corinthians.
We petition Our Lord that our sins may be forgiven, or dimissis peccatis vestris vel nostris, but it is through the grace of the Sacraments as noted above, that there is actual remission and absolution.
The True Mass, being the expression of the Catholic Faith, is the means through which certain sacramental graces are bestowed.