A general confession is a review of all of ones sins in one’s lifetime. Part of the motivation is to confess sins which one may have overlooked in past confessions–but those forgotten/overlooked sins would have been absolved anyhoww, unless one deliberately ommited a sin out of embarrassment or whatever.
The greater purpose of the exercise is, I believe, to remind oneself of the fact that one is a sinner in constant need of grace, to keep before one’s eyes the degree to which one falls short of the optimal standard which God would have one to live by.
And, perhaps, to give oneself a renewed sense of God’s mercy and love, to realise more perfectly how marvelous is God’s love for us despite ourselves.
The (formerly) Father Corapi preached a sermon on this some years before he left the Priesthood and while I was discerning whether or not to return to the Church. He is perhaps the only modern priest I have come accross who commended the practice of doing a general confession regularly. His argument, as I recollect, was that while past generations of Catholics often suffered from a high degree of scrupulosity (the anxiety that one’s sins have not been forgiven owing to some perceived defect in their confessions), modern Catholics suffer more often from the opposite, and tend to treat sin and confession too lightly.
BTW-the mention of (formerly) Fr. Corapi is not an endorsement of any of his current activities: I am uncertain of the details of his departure from EWTN and from the priesthood, but believe he has been somewhat rebellious since that time.
Hope this helps. Thanks for the responses thus far!