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Our Lady of Fatima told the 3 children that she wished for a short prayer to be added at the end of each decade of the Rosary: 
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O my Jesus, forgive us our sins, save us from the fires of hell; lead all souls to heaven, especially those most in need.
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With popular usage over the years, the words "...of Thy mercy" have been appended at the end.
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I was at a Rosary and Requiem Mass for a deceased Catholic recently, when the priest leading the Rosary omitted the Fatima Decade prayer, entirely. Perhaps this is normal for such Rosaries at the occasion of the Requiem for the dead and I hadn't noticed, as is the omission of the Glory Be prayer for which, "Eternal rest grant unto him, O Lord, and may perpetual light shine upon him. May his soul and all the souls of the faithful departed rest in peace. Amen," is said instead.
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Nonetheless, since I am so accustomed to including the Decade Prayer after each Glory Be, it was a matter of distraction for me to hear it left out after each decade. I don't need much to distract me during a Rosary anyway, so this is not a surprise that I would be distracted so easily. Be that as it may, I was left thinking about the Decade Prayer all during the Mass, and consequently, I discovered that the words Our Lady wished for us to add express intentions that are found in nearly every prayer proper of the Requiem Mass.
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In case you'd like to see what I mean, I'll put them here so you don't have to look them up. 
This is from the "Mass and Absolution on the Day of the Death or on the Day of the Burial of One of the Faithful," as found in the 1945 Fr. Lasance Missal beginning on pg. 1518.
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Note: the following responsory, if it is a Solemn Mass, is said by the priest and his ministers when they meet the body of the deceased inside the entrance to the Church.
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Come to his (or her, typical throughout) assistance, ye saints of God! Meet him ye angels of the Lord. Receive his soul and present it to the Most High.
V. May Christ who called thee, receive thee; and may the angels lead thee into the bosom of Abraham.
R. Receive his soul, offering it in the sight of the Most High.
V. Eternal rest grant to him, O Lord, and let perpetual light shine upon him.
R. Offering it in the sight of the Most High.
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This recalls the ending of the Decade Prayer, lead all souls to heaven, especially those most in need.
It is specifically applied to the particular soul for whom this Mass is being offered, who is "most in need" of our prayers.
It seems to me that making this connection between the Decade Prayer and the Mass prayers is a great help in contemplation and meditation on the prayers, which helps to battle against distraction because it gives us more to think about.
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Introit:
Eternal rest give to them, O Lord; and let perpetual light shine upon them. (4 Esdr. 2: 34, 35)
(Ps. 64: 2,3) A hymn, O God, becometh Thee in Sion; and a vow shall be paid to Thee in Jerusalem: O Lord, hear my prayer; all flesh shall come to Thee. Eternal rest ...
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Again, this recalls the ending of the Decade Prayer, lead all souls to heaven, especially those most in need.
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Prayer:
O God, Whose property is ever to have mercy and to spare, we humbly supplicate Thee for the soul of Thy servant, N., which Thou has this day called out of this world, Thou deliver it not to the hands of the enemy, nor forget it forever, but command it to be received by the holy angels and taken to Paradise, its home, so that, since it hath hoped and believed in Thee, it may not bear the pains of hell, but possess everlasting joys, Through our Lord Jesus Christ, Thy Son, Who liveth and reigneth with Thee in the unity of the Holy Ghost, God, world without end. Amen.
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This recalls more of the Decade Prayer, O my Jesus ... save us from the fires of hell; lead all souls to heaven, especially those most in need, and it also touches on the aspect of mercy in the popularly appended "of Thy mercy."
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The Epistle (I Thess. 4: 13-18 ) is along the same lines but I'll omit it here for brevity's sake. Just the Propers!
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Gradual:
Eternal rest give to them, O Lord; and let perpetual light shine upon them.
V. (Ps. 111:7) The just shall be in everlasting remembrance; he shall not fear the evil hearing.
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This recalls not only the Decade Prayer but the Prayer above, "Thou deliver it not to the hands of the enemy, nor forget it forever,..." 
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Tract:
O Lord, absolve the souls of all the faithful departed from every bond of sin. 
V. And to enjoy all the beatitude of the light eternal.
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This recalls, finally, that part of the Decade Prayer that begs for God's remission of our sins: O my Jesus, forgive us our sins, save us from the fires of hell, so as to effect our eternal salvation:  lead all souls to heaven...
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Sequence:
The  Dies Irae  is then recited, which is practically a thread all its own, so I'll skip that. Let me know if you miss it! 
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Offertory:
O Lord Jesus Christ, King of glory, deliver the souls of all the faithful departed from the pains of hell and the deep pit; deliver them from the lion's mouth, that hell engulf them not, nor they fall into darkness, but that Michael, the holy standard-bearer, bring them into the holy light which Thou once didst promise to Abraham and his seed.
V. We offer Thee, O Lord, sacrifices and prayers of praise; do Thou accept them for those souls whom we this day commemorate; grant them O Lord, to pass from death to the life which Thou once didst promise to Abraham and his seed.
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This recalls not only the Decade Prayer (requested of us by Our Lady) but also Our Lady's Magnificat, where she says, "...As he spoke to our Fathers: to Abraham and to his seed forever."
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Secret:
Be merciful, we beseech Thee, O Lord, to the soul of Thy servant, N., for which we offer to Thee the sacrifice of praise, supplicating Thy majesty that, through these offices of pious propitiation it may be worthy to enter unto everlasting rest. Through our Lord (Jesus Christ, Thy Son, etc.).
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This recalls that part of the Decade Prayer, O my Jesus,... save us from the fires of hell; lead all souls to heaven,... and it includes the popularly appended "...of Thy mercy" as well, beseeching Our Lord to be merciful specifically to the soul of the faithful departed for whom this Mass is being offered.
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Communion:
May light eternal shine upon them, O Lord: with Thy saints forever, for Thou art kind. (4 Esdr. 2: 35, 34)
V. Grant them everlasting rest, O Lord, and let perpetual light shine upon them. With Thy saints forever, for Thou art kind.
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Postcommunion:
Grant, we beseech Thee, O almighty God, that the soul of Thy servant, N., which hath today departed this life, being purged by this sacrifice and rid of sins, may obtain alike pardon and everlasting rest. Through our Lord (Jesus Christ, Thy Son,...).
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This Postcommunion prayer recalls the entire Fatima Decade Prayer, giving it yet a new emphasis, if that be possible: O my Jesus, forgive us our sins, save us from the fires of hell; lead all souls to heaven, especially those most in need.
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Absolution of the Corpse:
Enter not into judgment with Thy servant, O Lord, for in Thy sight shall no man be justified, unless remission of all sins be accorded him (her) by Thee. We beseech Thee, therefore, that Thy judicial sentence weigh not heavily upon him (her) who is commended to Thee by the true supplication of the Christian faith, but, with the help of Thy grace, may he (she) be worthy to escape the sentence of vengeance, seeing that, while he (she) lived he (she) was sealed with the seal of the Holy Trinity. Who livest and reignest world without end. 
R. Amen.
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Once again, this recalls the entire Decade Prayer, giving it yet another emphasis: O my Jesus, forgive us our sins, save us from the fires of hell; lead all souls to heaven, especially those most in need.
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Responsory (Libera Me):
R. Deliver me, O Lord, from eternal death on that dreadful day, when the heavens and the earth shall be moved, and Thou shalt come to judge the world by fire. 
V. I am seized with fear and trembling, when I reflect upon the judgment and the wrath to come.
R. When the heavens, and the earth shall be moved.
V. That day a day of wrath, of wasting and of misery, a dreadful and exceeding bitter day.
R. When Thou shalt come, to judge the world by fire.
V. Eternal rest grant unto him, O Lord, and let perpetual light shine upon him.
R. Deliver me, O Lord, from eternal death on the dreadful day, when the heavens and the earth shall be moved and Thou shalt come to judge the world by fire.
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Note: this Libera Me (along with the somewhat similar Dies Irae, which contains these same themes) has become omitted by the Novus Ordo ritual in common use for the burial of the dead, since they make people feel "uncomfortable." Can't have that!
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In any event, this totally encapsulates the pithy Decade Prayer, expanding it to a much more complete and meaningful supplication, for those who would open their eyes and seeing, they may see. IOW once the expanded prayers are read, understood, and CONNECTED to the Decade Prayer, every time we say it during our Rosary it will recall to mind all these longer and more completely filled-out prayers which will demand our attention and thus do battle against DISTRACTIONS. By this act of our will we cooperate with God's grace and receive therefore the according increase of virtue.
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O my Jesus, forgive us our sins, save us from the fires of hell; lead all souls to heaven, especially those most in need.
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Furthermore, reading through these propers and meditating on their relation to the Decade Prayer, it's not too difficult to comprehend that Our Lady's ending with "those most in need" (and omitting "...of Thy mercy"), shows us that some of us are in need of something OTHER than the mercy of God. The Blessed Virgin Mary asked the children for us to pray especially for those in need, but she did not specify in WHAT they are in need. Perhaps they are in need of humility, or contrition, or penance, or self-denial, or mortification, or virtue. Since fear of God is the beginning of wisdom, so maybe they're in most need of Fear of the Lord! If one is confirmed validly, one has received this gift of the Holy Ghost but that doesn't mean that one must therefore have ACCEPTED the gift and PUT IT INTO PRACTICE. Maybe pride gets in the way, for example. 
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Perhaps those most in need of CONVERSION to the ONE TRUE FAITH is what we should pray for, such as certain present day clerics (who shall here remain nameless). For God's mercy, while infinite, is not to be short-changed by the efficacious change of heart that we can effect in our own soul by an act of our will. God respects the human will, and granting to us whatever mercy He may bestow upon us, if we remain steadfast in opposition to His divine graces we are not necessarily to be overpowered by such mercy. Our own conversion, or change of heart, by cooperating with God's grace and subjecting ourselves to His will, can facilitate a more profound and complete virtue than His mercy alone can do.
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