The desire to take advantage of the sacrament of reconciliation which is also called the sacrament of confession, or the sacrament of penance is admirable. Sadly, many people neglect this great means of receiving God’s grace.
First of all, You must not wait to have a mortal sin in order to go to confession. It isn’t necessary. One of the fruits of frequent reception of the sacrament, in fact, is to help us avoid sin and especially mortal sin. In that case, it is enough to confess venial sins, if that is all a person is aware of having committed.
To back this up, The Catechism of the Catholic Church, in No. 1458, states, “Without being strictly necessary, confession of everyday faults (venial sins) is nevertheless strongly recommended by the Church. Indeed the regular confession of our venial sins helps us form our conscience, fight against evil tendencies, let ourselves be healed by Christ and progress in the life of the Spirit.” A confession is valid even if someone doesn’t mention all his venial sins.
It is also is valid still, if one mentions a mortal sin from the past, so long as the sin was previously confessed and absolved. A penitent does, however, have to mention mortal sins that he forgot to confess previously.
Note that; If a penitent deliberately holds back or does not mention a mortal sin he committed, then the sacrament is invalid and the sin of sacrilege is added to the guilt.
Receiving the Sacrament weekly is praiseworthy, especially for a lay person. In many religious congregations, it is the recommended norm. In principle, a person could approach the sacrament even more frequently. But here it is important that a person isn’t doing this because of scrupulosity. Some basically good people might fear that the sins from the last confession weren’t forgiven. Or they might think that they are constantly falling into mortal sin and need to go to confession every few days. A person who commits a mortal sin should, of course, try to go to confession as soon as possible. In most cases, though, weekly confession suffices very well for a lay person with a healthy spiritual life.
Although mentioning a mortal sin already forgiven can lead a penitent to a deeper appreciation for God’s mercy, we should be cautious. If mentioning past sins brings undue anxiety, or if it springs from scrupulosity, that is not good. Therefore, it is better to leave such sins in the past. Jesus doesn’t want us to beat up on ourselves needlessly. He completely forgets our past sins that we have sincerely confessed, and he wants us to do the same.
Also, someone who feels he needs to go to confession every few days might want to consider doing so with the same priest. That would give the confessor a chance to know the penitent and to look for signs of scrupulosity or other problems that might require special attention.
Finally, and although this is optional, a penitent is free to mention temptations. Mentioning temptations can help in different ways. First, it might bring comfort to the penitent, who comes away from the sacrament with an even deeper sense of opening his soul to Christ. Secondly, mentioning a temptation can help weaken its power, since talking openly about a problem can be half its solution. Also, it gives the confessor a chance to recommend additional aids to the penitent.