It's good that you know the three criteria required to commit a mortal sin:
- it must be grave matter
- there must be sufficient knowledge, and
- it must be done with full consent of the will.
RE: Criteria for a mortal sin from the Catechism: CCC 1857 — 1860
1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."
1858 Grave matter is specified by the Ten Commandments, corresponding
to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother." The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.
1860 Unintentional ignorance can diminish or even remove the imputably of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The prompting's of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.
Mine sin involved grave matter but I did not give it any reflection at the time. Anyway, about 15 years ago I did go to Confession for it and I told the priest this sin occurred 20 years prior, but I cannot remember if I told him that during those 20 years, I had been to Confession several times and had not confessed it as a mortal sin due to embarrassment.
It is unclear whether you thought you had committed a mortal sin or not. If you had not, because there was no reflection or full consent, you have nothing to worry about.
On the other hand, if you have any concern that all criteria above were met, Yes!, you should tell the Confessor in your next Confession what mortals sins you have withheld in the past . . . even if you just think they were mortal sins. (Go with what your gut tells you; that gut feeling is called the natural law. The natural law is written on everyone's soul and tells them what is right and wrong without anyone else telling them it is right or wrong.)
The Seal of the Confession is absolute and, in my opinion, your embarrassment to tell the priest what sins you committed is a form of pride on your part. Don't worry!
The Catechism tells us the basis structure we have had from generation to generation:
1448 Beneath the changes in discipline and celebration that this sacrament has undergone over the centuries, the same fundamental structure is to be discerned.
It comprises two equally essential elements:
- on the one hand, the acts of the man who undergoes conversion through the action of the Holy Spirit: namely, contrition, confession, and satisfaction;
on the other, God's action through the intervention of the Church. The Church, who through the bishop and his priests 1.) forgives sins in the name of Jesus Christ, 2.) determines the manner of satisfaction, and 3.) also prays for the sinner and does penance with him.
Thus the sinner is healed and re-established in ecclesial communion.
It goes on to say:
VII. The Acts of the Penitent
1450 "Penance requires . . . the sinner to endure all things willingly, be contrite of heart, confess with the lips, and practice complete humility and fruitful satisfaction." (Roman Catechism II,V,21; cf. Council of Trent (1551): DS 1673)
1451 Among the penitent's acts contrition occupies first place. Contrition is "sorrow of the soul and detestation for the sin committed, together with the resolution not to sin again." (Council of Trent (1551): DS 1676)
1452 When it arises from a love by which God is loved above all else, contrition is called "perfect" (contrition of charity). Such contrition remits venial sins; it also obtains forgiveness of mortal sins if it includes the firm resolution to have recourse to sacramental confession as soon as possible. (cf. Council of Trent (1551): DS 1677)
1453 The contrition called "imperfect" (or "attrition") is also a gift of God, a prompting of the Holy Spirit. It is born of the consideration of sin's ugliness or the fear of eternal damnation and the other penalties threatening the sinner (contrition of fear). Such a stirring of conscience can initiate an interior process which, under the prompting of grace, will be brought to completion by sacramental absolution. By itself however, imperfect contrition cannot obtain the forgiveness of grave sins, but it disposes one to obtain forgiveness in the sacrament of Penance. (cf. Council of Trent (1551): DS 1678; 1705)
1454 The reception of this sacrament ought to be prepared for by an examination of conscience made in the light of the Word of God. The passages best suited to this can be found in the Ten Commandments, the moral catechesis of the Gospels and the apostolic Letters, such as the Sermon on the Mount and the apostolic teachings. (cf. Matthew 5-7; Romans 12-15; 1 Corinthians 12-13; Galatians 5; Ephesians 4-6; etc.)
Stop by and have the priest hear your Confession this Saturday; if you are struggling with something during the Confession ask him for help and, to put your mind at ease, end every Confession the way I do. I say:
I just want to make a good, holy Communions.
You can't lose by ending any Confession that way if your repentance includes the following:
1490 [A] movement of return to God, called conversion and repentance, entail[ing]:
- sorrow for and abhorrence of sins committed, and
- the firm purpose of sinning no more in the future.
Conversion touches the past and the future and is nourished by hope in God's mercy.
If you need help with this, search out a good spiritual director: a priest who can counsel and assist you on a regular basis.
Finally, remember we always beat the heck out of Satan when we live a prayerful, sacramental life and receive the sacraments in a state of grace.