Assuming that the portion of St. Faustina’s diary that you read was read correctly and in its proper context (something I am unable to verify without a quote from the diary), there are several things to keep in mind:[LIST]
*]The diary of the apparitions received by St. Faustina contains private revelation and, as such, it is not incumbent upon any Catholic to believe in it. Because the Church has approved of the private revelation given St. Faustina, it has been offered to the faithful for belief but Catholics are free to set it aside.[/LIST][LIST]
*]As the original diary was written in Polish, it is possible that you were reading a faulty translation. (Indeed, the Divine Mercy devotion was unapproved by the Church for many years because of problematic translations of the original diary.)[/LIST][LIST]
*]Canonized saints have occasionally been mistaken on points of the faith. For example, the theological giant St. Thomas Aquinas was unable to reconcile the Immaculate Conception with Christ’s universal redemption of mankind. Aquinas himself said:[/LIST]
f I have written anything erroneous … I submit all to the judgment and correction of the holy Roman Church, in whose obedience I now pass from this life (source).In short, if the Church teaches a particular point of the faith that appears to be disputed by a private revelation, even an approved private revelation, one should defer to the Church. In this case, honestly forgotten sins are forgiven because human beings are not held culpable for what they do not will. If they confess with the intent to reveal all that must be revealed but sincerely forget something, they will still be forgiven. If they later remember a mortal sin, they should then mention it in their next confession.
Understanding Scrupulosity** by Fr. Thomas M. Santa, C.Ss.R.