If I truly mean it then they are forgiven.
Go and reread what happened to David after he adultered with Bathsheba (from memory it’s in either 1 or 2 Samuel, chapters 12 and 13).
Certainly David fasted, did penance and prayed for forgiveness. Do you think he meant it? Of course. No-one who reads Psalm 51 ("Have mercy on me, Lord, according to your lovingkindness …’), supposedly written at this time, can doubt his sincerity.
Did he then say to himself ‘I truly meant it, therefore I know my sins are forgiven’ and leave it at that? Not a bit. He confessed to God’s representative - the prophet Nathan - saying ‘I have sinned against the Lord’. He didn’t give details, because on that occasion he didn’t need to, since it was Nathan who had pointed out David’s sin to him.
Then he waited until NATHAN pronounced absolution - ‘For your part the Lord has forgiven your sin’. But Nathan did more. He also announced the penance God would require of David - that the child he and Bathsheba had adulterously conceived would die.
How do you know your sins are forgiven?
Because of the COMBINATION, the one David relied on, of my true repentance and the priest’s pronouncement of absolution.
How does the priest know you are truly sorry for your sins?
God has His ways. He showed Peter that Annias and Sapphira weren’t truly sorry, didn’t He?
Then there’s the fact that I actually went to the effort of either looking up the church timetable and going there at the time scheduled for confessions, or phoning the priest and making an appointment with him in order to confess outside the scheduled times. Then I have to examine my conscience - think individually about and name ALL my serious sins to the priest.
This effort indicates the seriousness with which I view both my sins and the obligation to confess them. It’s something incredibly few people if any would bother doing on a whim or half-heartedly, if they weren’t truly repentant.
What effort really needs to be expended, what serious thought and contemplation needs to be contributed, to a simple prayer of ‘God, I’m sorry’? How often do we say similar things to people we bump against in the street or on whose toes we accidentally tread, or whose phone number we accidentally dial? How seriously do we think about those apologies, mean them wholeheartedly, form real resolutions never to do the like again?
For serious sin we owe God much more than that.