I will try to deal with some aspects of sin but my response cannot be all-inclusive. Before I begin to answer this question, I want to emphasize that God is not an ogre waiting to get us. God is love. God loves us and is always there waiting to help us - "I stand at the door and knock." I want you to read this response in that light.
Yes, the Church requires Confession before Communion for serious or mortal sins, "unless one has a grave reason for receiving Communion and there is no possibility of going to Confession" (Catholic Catechism, n. 1385). Many of us used to think that every little thing was a serious sin, so we only went to Communion if we could go to Confession just before Mass.
The priest would come to the school on Friday to hear the children's Confessions so he'd have more time on Sunday for adult Confessions. However, on Sunday morning, the children would rush to Confession because they saw every little misdemeanour as serious, preventing reception of Communion.
The first question we need to ask ourselves is what we understand by sin. Sin can be defined in different ways but basically it is any action or omission which fails in our duty towards God, our neighbour or ourselves. It is a turning away from God and good; it is a break in our relationship with God. When we sin we fail to follow the conscience that God has put into our hearts.
The Church has distinguished between serious sin, that is, mortal sin and lesser transgressions or venial sin, although today, we do not hear these terms used often. There have always been, in the Church's view, three requirements to make a sin mortal.
First, the matter must be grave or serious. Otherwise, an offence against God or another may be sinful but not mortally sinful. What is serious matter? Direct acts against the Ten Commandments which basically list our duty towards God, others and self, would fall under this category. However, infractions such as doing minor injury to someone would not be serious matter, although they would still be wrong.
Second, the person committing the offence must have full knowledge, that is, must fully understand that the action is seriously wrong. Thus, young children or severely mentally challenged individuals are not considered to have sufficient knowledge and understanding to commit mortal sin.
Third, the person must give full consent of the will, fully and freely agreeing to do it, knowing its seriously wrongful nature. Sins of passion or human weakness generally would not fall into this category.
Now, let us look at the sinfulness of missing Mass on Sundays. The Church considers this to be a serious matter, since the Mass is the summit to which our whole Christian life tends. As a Christian community, we, the Body of Christ, the people of God, gather to celebrate the love God has bestowed upon us.
We, who live in Christ, with Christ and for Christ come together in praise and thanksgiving to celebrate the gift of redemption and the life of grace received from God. We come to follow Jesus' command "Do this in remembrance of me" and to partake of his body and blood offered for us. This is something we do especially on Sunday, a day made sacred by Christ's resurrection.
Since missing Mass is a serious matter, one's culpability would need to be judged on the basis of the other two aspects of a serious sin. Why did you miss Mass? Was it a deliberate and fully knowledgeable act without any mitigating circumstances? Or were you ill? Was it due to fatigue from late partying on Saturday? But that can't be an excuse any longer since Mass is available on Saturday and as well as various times on Sundays.
Did commitments such as work or assignments, etc., prevent you from taking time for Mass? Many of us are so busy all week that when Sunday comes we just don't have the energy to keep on. There are so many things in today's society that make it hard to be faithful to our Christian commitments.
Therefore, one needs to look inside oneself to determine the degree of one's sinfulness and one's preparedness to receive the Body and Blood of Christ. No one else can do this for you.
Perhaps this is good wake-up call for us to examine our busy lives and see where they are leading.
Do we take time to nourish our faith and our human life? Or do we expend so much energy on work and responsibilities that we're just too worn out to develop closer relationships with God and others?
As we take stock, one thing we must never forget no matter what we have done is that God is a God of love and mercy, not a vengeful and punishing God. As the Gospels point out so well, Jesus came to save, not to condemn us.