217 Q. What must he do who has willfully concealed a mortal sin in confession? A. He who has willfully concealed a mortal sin in confession must not only confess it, but must also repeat all the sins he has committed since his last worthy confession.
“Willfully.” Remember, forgetting is not the same as concealing; but if you should willfully neglect to examine your conscience or make any effort to know your sins before going to confession, then forgetting would be equivalent to concealing. Without any preparation your confession could hardly be a good one. When you are in doubt whether an action is sinful or not, or whether you have confessed it before, you should not leave the confessional with the doubt upon your mind.
It is a foolish practice, however, to be always disturbing your conscience by thinking of past sins, especially of those that occurred very early in your life. Sometimes it is dangerous; because if, while thinking of your past sins, you should take pleasure in them, you would commit a new sin similar to the past sins in which you take delight.
It is best, therefore, not to dwell in thought upon any particular past sin with the time, place, and circumstances of its commission; but simply to remember in general that you have in the past sinned against this or that Commandment or virtue.
The past is no longer under our control, while the future is, and becomes for us, therefore, the all-important portion of our lives. Not unfrequently it may be an artifice of the devil to keep us so occupied with past deeds that we may not attend to the dangers of the future. Do not, then, after your confession spend your time in thinking of the sins you confessed, but of how you will avoid them in the future. When a wound is healed up, nobody thinks of opening it again to see if it has healed properly; so when the wounds made in our souls by sin are healed up by the absolution, we should not open them again.