How do you make a proper confession?

I’m not sure how to word this question, but I’ll try my best. If a penitent leaves out what he or she believes to be venial sins in confession, but later learns that what he or she thought was a venial sin is a mortal sin; is he or she morally obligated to confess those sins in a confession before again receiving the Eucharist? Also, if so, is he or she required to state the number of times the sin was committed even if it was a repeat sin over many years? For example, many people do not know that missing Mass on Sunday & Holy Days (through their own fault) is a mortal sin, later they may learn it is a mortal sin and they may not know exactly how many Sunday & Holy Day Masses they missed but may know the number of months or years they didn’t bother to go to Mass. Would it be sufficient to state in Confession something like “For 5 years I only went to Mass on Easter & Christmas.” or would the penitent be required to determine exactly how many Sundays & Holy Days were missed during the 5 years, and state something like "I did not bother to atttend 285 Masses of Obligation (or whatever the number would be)?

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Recommended Reading:

Unknown Mortal Sin now known
Why is it important that I confess the number of times I have committed a sin?
How specific should I be during confession?

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