If someone goes to confession and confesses everything they remember, but they truly forget one sin, which happens to be a mortal sin, is that sin forgiven or do you need to confess it during your next confession?
Q. 793. Is our Confession worthy if, without our fault, we forget to confess a mortal sin?
A. If without our fault we forget to confess a mortal sin, our Confession is worthy, and the sin is forgiven; but it must be told in Confession if it again comes to our mind.
Q. 794. May a person who has forgotten to tell a mortal sin in confession go to Holy Communion before going again to confession?
A. A person who has forgotten to tell a mortal sin in confession may go to communion before again going to confession, because the forgotten sin was forgiven with those confessed, and the confession was good and worthy.
Yes, the sin is forgiven but it should be confessed at the next reasonable opportunity for confession.