Is the sacrament of confession only necessary when you have a mortal sin?

Yes, that is why it was created, but that does not mean you cannot use it for venial sins, because going to the confessional gives you graces you do not get by confessing venial sins on your own, and that is an incentive to go. Only through the confessional can a mortal sin be forgiven in the Catholic Church. The one exception, according to the Council of Trent, is the desire of confession in an emergency, for example if your plane is going down and there is no priest; you can make an act of contrition with the desire to confess, just as there is baptism of desire in danger of death. The Church is not merely black and white; she makes provision for the contingencies of life, especially death.