Mrs. Don-o is correct in that she ought to mention the past unworthy reception(s) in formal confession, as we all would other transgressions that come to mind. It is not necessary to be O-C in one’s appraisal of one’s own confession history, but neither should one deliberately omit sins assuming they have already been forgiven. I was told this by a priest directly. It is quite appropriate to conduct a full lifetime examination of conscience periodically, in case forgotten or repressed sins are uncovered.
None of this asserts that God has not already forgiven the sins, but to miss an opportunity to confess a given sin in the Sacrament of Confession is to forgo the grace associated with forgiving of that sin.