The Leonine prayers first included three Hail Marys, short response, and a concluding prayer. The prayer of Saint Michael was written and added to those "prayers after Mass" by Pope Leo two years later, and finally Pope Pius X allowed the option of adding "Sacred Heart of Jesus, have mercy on us" three times at the conclusion.
Since the Leonine prayers were to be said after a "low Mass," that is, a Mass where there is no singing of the prayers done by the priest, I thought it fitting to continue the prayer of Saint Michael after the weekday Masses and not the Sunday Masses. As you know, Sundays at our parish include much singing because they are always solemnities, the highest and most important form of the Mass, which includes three readings and the Creed and the Gloria (except for Advent/Lent). Because we are singing at these Masses, the Leonine prayers would not have been said from 1884-1962, or at least were not required. We will continue to pray for our church, especially for the continued conversion of bishops and priests, at weekday Masses.
So, on Sundays at our parish, we will not use the prayer of Saint Michael after the "Go in peace," but you are welcome and encouraged to continue the prayer as part of your personal prayers after Mass. Below I offer this prayer, and another beautiful prayer, as options for your use during your prayer of thanksgiving.
St. Michael Prayer
Saint Michael Archangel, defend us in battle,
be our protection against the wickedness and snares of the devil;
may God rebuke him, we humbly pray;
and do thou*, O Prince of the heavenly host, [* or "and you"]
by the power of God, cast into hell Satan and all the evil spirits
who prowl through the world seeking the ruin of souls.
Prayer after Communion, by Saint Thomas Aquinas
I thank You, Lord, Almighty Father, Everlasting God, for having been pleased, through no merit of mine, but of Your great mercy alone, to feed me, a sinner, and Your unworthy servant, with the precious Body and Blood of Your Son, our Lord Jesus Christ. I pray that this Holy Communion may not be for my judgment and condemnation, but for my pardon and salvation. Let this Holy Communion be to me an armor of faith and a shield of good will, a cleansing of all vices, and a rooting out of all evil desires. May it increase love and patience, humility and obedience, and all virtues. May it be a firm defense against the evil designs of all my visible and invisible enemies, a perfect quieting of all the desires of soul and body. May this Holy Communion bring about a perfect union with You, the one true God, and at last enable me to reach eternal bliss when You will call me. I pray that You bring me, a sinner, to the indescribable Feast where You, with Your Son and the Holy Spirit, are to Your saints true light, full blessedness, everlasting joy, and perfect happiness.
Through the same Christ our Lord. Amen.