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For a valid confession, one must confess all his mortal sins (if he has any). One is not required to confess venial sins; they can be remitted other ways, such as by devoutly receiving the Eucharist.* However, attachment to venial sins predisposes oneself to commit mortal sin.
*cf. the ยง "The Eucharist Remits Venial Sins" of the Catechism of the Council of Trent on the Eucharist
At least imperfect contrition (attrition) for one's sins is required for a valid confession. Fr. Hardon, S.J., defines imperfect contrition as
Sorrow for sin animated by a supernatural motive that is less than a perfect love of God.
From the Catechism of the Council of Trent, chapter on penance, concealing sins in confession is a serious matter:
SINS CONCEALED
So important is it that Confession be entire that if the penitent confesses only some of his sins and willfully neglects to accuse himself of others which should be confessed, he not only does not profit by his Confession, but involves himself in new guilt. Such an enumeration of sins cannot be called sacramental Confession; on the contrary, the penitent must repeat his Confession, not omitting to accuse himself of having, under the semblance of Confession, profaned the sanctity of the Sacrament.