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1) Unlike a particular (ie., regular) confession, it is not necessary to confess all your sins in a general confession, if you have already confessed them before. Some people use a general confession just to confess a handful of their most humiliating or most serious lifetime sins;
This is not correct. The first general confession must be for sins committed since attaining the age of reason or since baptism. Any general confessions after that should begin with sins committed since the previous general confession.
You cannot just "pick and choose" during a general confession. Otherwise, this is simply a normal confession where you are confessing some past sins.