Is somebody who was christened a Catholic but didn't complete their Confirmation/First Holy Communion still allowed to go to confession with a Catholic priest to confess a sin, repent and ask for forgiveness?
No, Confirmation is not necessary for being able to receive absolution in Confession.
[actual] sins which are destroyed by Penance may properly be called the matter of Penance.
Thus, if a person has actual sins (sins he commits, not Original Sin), he can be absolved of them in Confession.
Must you have completed your confirmation to be able to go to confession?
The short answer is no.
Normally, the reception of first confession predates both first communion and reception of the sacrament of confirmation...
Receiving the Sacrament of Penance before receiving First Holy Communion is not optional, but rather normative. In approaching this subject, keep in mind that the Church has a good reason for any rule, and for this one also.
Remember that until this century, the reception of the Sacraments of Penance and Holy Eucharist occurred when a person was usually a teenager. However, on August 8, 1910, Pope St. Pius X issued the decree Quam singulari which permitted a child, who has attained the age of reason (“at about the seventh year, give or take something”), to receive these sacraments. While the Holy Father definitely wanted children to receive the Holy Eucharist, he also appreciated the intrinsic relationship between this sacrament and Penance.
Pope St. Pius X understood that a child’s moral conscience begins to develop with his ability to reason. Children can know right from wrong, the meaning of the commandments, and the nature of sin, mortal and venial. Frankly, if children can in the simplest way understand the profound mystery of the Holy Eucharist, they certainly can understand the notion of sin and repentance. Consequently, he underscored the need for the Sacrament of Penance: “The custom of not admitting to confession children who have attained the use of reason, or of never giving them absolution, is to be totally condemned” (Quam singulari).
Placing First Penance before First Communion is prudent from a catechetical perspective. Children are taught the great love of God for each of us, especially in the fundamental belief that Jesus, true God who became true man like us in all things but sin, suffered, died, and rose to forgive our sins and grant us salvation. Through baptism, we enter into this saving mystery, and we struggle to live our baptism through prayer, worship, good works, and obedience to God’s commandments. Yet, at times we freely choose to sin. Just as a child understands that “breaking” his parents’ rules offends them and incurs punishment, so a child can understand the consequences of “breaking” God’s rules. We trust, however, in the infinite love and mercy of God which is shown to each of us in the Sacrament of Penance. In this sacrament, we repent of our sins with sincere contrition, confess them, and receive absolution. Through regular confession, we are safeguarding the presence of our Lord in our souls in sanctifying grace and are preparing for our ultimate union with the Lord.- Is receiving First Penance before First Holy Communion optional?
In order to receive Confirmation one must be in a state of grace. One should receive the sacrament of Penance in order to be cleansed for the gift of the Holy Spirit. More intense prayer should prepare one to receive the strength and graces of the Holy Spirit with docility and readiness to act under His influence.
The Catechism of the Catholic Church makes it clear that the the sacrament of confession should be administered before reception of the sacrament of confirmation.
1306 Every baptized person not yet confirmed can and should receive the sacrament of Confirmation. Since Baptism, Confirmation, and Eucharist form a unity, it follows that "the faithful are obliged to receive this sacrament at the appropriate time," for without Confirmation and Eucharist, Baptism is certainly valid and efficacious, but Christian initiation remains incomplete.
1307 For centuries, Latin custom has indicated "the age of discretion" as the reference point for receiving Confirmation. But in danger of death children should be confirmed even if they have not yet attained the age of discretion.
1308 Although Confirmation is sometimes called the "sacrament of Christian maturity," we must not confuse adult faith with the adult age of natural growth, nor forget that the baptismal grace is a grace of free, unmerited election and does not need "ratification" to become effective. St. Thomas reminds us of this:
Age of body does not determine age of soul. Even in childhood man can attain spiritual maturity: as the book of Wisdom says: "For old age is not honored for length of time, or measured by number of years. "Many children, through the strength of the Holy Spirit they have received, have bravely fought for Christ even to the shedding of their blood.
1309 Preparation for Confirmation should aim at leading the Christian toward a more intimate union with Christ and a more lively familiarity with the Holy Spirit - his actions, his gifts, and his biddings - in order to be more capable of assuming the apostolic responsibilities of Christian life. To this end catechesis for Confirmation should strive to awaken a sense of belonging to the Church of Jesus Christ, the universal Church as well as the parish community. The latter bears special responsibility for the preparation of confirmands.
1310 To receive Confirmation one must be in a state of grace. One should receive the sacrament of Penance in order to be cleansed for the gift of the Holy Spirit. More intense prayer should prepare one to receive the strength and graces of the Holy Spirit with docility and readiness to act.
1311 Candidates for Confirmation, as for Baptism, fittingly seek the spiritual help of a sponsor. To emphasize the unity of the two sacraments, it is appropriate that this be one of the baptismal godparents.
Have you ever gone to confession before? If yes, then you definitely can go. If not, best to just ask the priest. This might seem to defeat the purpose of a private confession, but you'll probably want extra time if it's your first confession and you're older than eight years old. Making an appointment (the priest's number is usually in the church bulletin) is the best way to go for something like this.