It’s just occured to me. What can we do to get the most of the mass when for various reasons we can’t go to Communion?
If the reason is, let’s say, a mortal sin(s), do we have to repent, say to God that we’re sorry, pray for his mercy and long for Him?
Yes, I know the answer - go to confession as soon as possible

But what can we do to avail ourselves of our presence at the mass when we can’t go and receive the Eucharist once we’re already there?
What do you do?
Laudatur Iesus Christus.
To a certain extent “[how] to get the most [out] of the [M]ass” is the wrong question. The Mass is primarily a sacrifice. It is something Christ Jesus gives to the Father. We join with Christ and offer ourselves with Christ to the Father in the Spirit.
The proper question might rather be “how do I ***give ***more completely in the Mass?”
One important step in worshiping more fully is to pay close attention to what is actually happening at the Mass.
We purify ourselves as best we can, so that we are worthy to offer sacrifice. As we enter, we wash away our venial sins with Holy Water, renewing our Baptismal vows with the Sign of the Cross. This is the beginning of our re-commitment to God, our self-donation to the Trinity. We make an act of contrition and ask the Church, both those members present and those in Heaven, to pray for our forgiveness, that we might be worthy to offer ourselves as acceptable gifts to God.
During the Liturgy of the Word, we participate in a dialogue with God; He speaks to us through the texts of the Scriptures; we then respond by professing our Faith through the Creed, which distils the truths that the Scripture has made known to us. We then are bold enough to ask God for things; however, only things that are “in Christ’s name” can be asked. In other words, we display our understanding of the message God has given us, by asking him for and committing ourselves to seek the things that Jesus would deem worthy for us to desire and work for.
We then begin the sacrifice proper. We prepare ourselves mentally to commit ourselves to God and to the pursuit of holiness. We “sacrifice” ourselves, by setting ourselves apart for God’s use; we offer ourselves, our lives, our intentions to the Holy Trinity. This is done by identifying with the grains of wheat that are gathered by human labor and baked into the bread offering, by identifying ourselves with the countless grapes that are gathered by human hands and made into the offering of wine. We also show our willingness to offer ourselves and our labor by giving some portion of the money we have made so that it can be committed to God’s use in and through the Church.
However, these offerings of ourselves and of our labors are not sufficient as a participation in the Mass. At best they represent our human efforts to live the Gospel message as we have grasped it and to give God what we have to offer. Even though this is all that we have – and frankly we rarely even rise to that level of giving, we tend to hold back some of what we have – but even if we by a supernatural grace were able to fully offer all that we have, as some Great Saints have done, this would be a poor offering to God. On our own we are unworthy to be God’s friends.
Jesus saves us from this shortcoming. As in the Temple sacrifice of old, our offering is made to suffer what the worshiper should suffer. The bread and wine representing all of the members of the Church are placed upon the Altar and they are totally changed. As bread and wine their natures are absolutely destroyed. They cease to be what they were and they become absolutely and completely the Body and Blood, Soul and Divinity of Christ Jesus. (This is what we hope for, for ourselves. In our will and our actions, our hearts and our minds, by participating in this miracle, by allowing ourselves to be offered in that bread and wine, we re-commit ourselves to our conversion to become “the New Man.” We seek to become Christ completely and to have all imperfections burned away from us, so that we can enter into communion with the Trinity.)
Continued . . .