Under normal circumstances, all serious sins that one remembers after making a thorough examination of conscience and that have not yet been confessed can be forgiven only in individual sacramental confession.
Of course there will be reluctance before making a confession. Overcoming it is the first step toward interior healing. Often it helps to think that even the Pope has to have the courage to confess his failings and weaknesses to another priest – and thereby to God. Only in life-or-death emergencies (for instance, during an airstrike in wartime or on other occasions when a group of people are in danger of death) can a priest administer “general absolution” to a group of people without the personal confession of sins beforehand. However, afterwards, one must confess serious sins in a personal confession at the first opportunity.
“According to the Church’s command, “after having attained the age of discretion, each of the faithful is bound by an obligation faithfully to confess serious sins at least once a year.” Anyone who is aware of having committed a mortal sin must not receive Holy Communion, even if he experiences deep contrition, without having first received sacramental absolution, unless he has a grave reason for receiving Communion and there is no possibility of going to confession. Children must go to the sacrament of Penance before receiving Holy Communion for the first time.” (CCC 1457)