Through Mary, the Eternal Word, who is the Son of God, entered time. This contact we now have with the Eternal reality is our salvation. In this sense, Mary is a mediatrix of eternity, but she isn’t eternal or "pre-existing," because her existence began when she was conceived. In fact, the notion of the pre-existence of any human soul (be it men, Christ, or Mary) is heresy.
Using correct terminology is important here.
From Saint Irenaeus to the Second Vatican Council, the Church has always taught that Mary cannot be separated from her Son, and that therefore she is part of the eternal plan of God who wanted the Incarnation to take place.
Being in God's plan does not mean existing in the bosom of God the Father, as it is the case for Jesus who is the eternal Word--"In the beginning was the Word, and the Word was with God, and the Word was God" (Jn 1:1-4). The Word pre-existed in his divinity; the human soul of Jesus did not pre-exist his conception.
That Mary was present in the thought of God from all eternity does not mean that her human soul was pre-existing.
Mary's existence had a beginning, just like ours. She was conceived and did not exist before that. The Church celebrates her conception on December 8th in the Latin liturgy (December 9th in the Eastern churches).
The answer is in the Sapiential books (Sirach, Proverbs) but again we need to have a rigorous approach.
The Bible says that Wisdom is a "pre-existing creature." And further:
"The LORD begot me, the beginning of his works, the forerunner of his deeds of long ago” (Proverbs 8:22).
"Before all ages, from the beginning, he created me, and through all ages I shall not cease to be" (Sirach 24:9).
If the Church wanted to affirm that Mary was a "preexisting" creature, she would have chosen the above readings for the mass of the Nativity of Mary (September 8th), or for the mass of the Conception of Mary (December 8th), or for the feast of Our Lady of Lourdes (since her highlighted title is the "Immaculate Conception").
But this is not the case.
There are, however, minor instances where the Roman Catholic Church chose these same texts in connection with the Virgin Mary, but again, we have to discern and apply nuance.
The feast of the Presentation of Mary (November 21st) does include Proverbs 8:22-31, but the Gospel reading is that of the coming of the shepherds to Bethlehem: so here the liturgy is oriented towards the birth of Jesus. This feast is connected to the motherhood of Mary, who was Jesus’ first temple. This passage from the Book of Proverbs has a Christological meaning.
The title "Mother of Hope" is a title given to Wisdom before being a Marian title.
Mary belonged to a people who had hope, in Hebrew "anawim." Hope preceded Mary’s existence and is an attribute of pre-existing Wisdom.
But now that Mary has been born, has lived on this earth, and then was transported to heaven in glory, she is the one who personally holds the title of "Mother of Hope."
We have to see our participation in the divine nature (2 Peter 1:4) in the larger divine scheme.
During our earthly life, all Christians, like Mary, are called to be participants in the divine life (2 Corinthians 1:4 and 1 Corinthians 9:23) and cooperators of God (1 Corinthians 3:9) because God is always active. Jesus said, "My Father is at work until now, so I am at work" (Jn 5:17).
After they are glorified, Mary and all the saints in heaven participate in the divine life and continue to cooperate with God. They participate in the knowledge of our origins, the origin of the universe, and the dynamism of the creative act, but always in a second position, without ever erasing the fact that we had a beginning, that we are creatures and not the Creator.
The pre-existence of souls (whether of men, of Christ, or of Mary) is a heresy that was condemned by the Second Council of Constantinople (and reiterated by Saint John Paul II ). Here is an authoritative statement from that council:
Whoever says or thinks that human souls pre-existed, i.e., that they had previously been spirits and holy powers, but that, satiated with the vision of God, they had turned to evil, and in this way the divine love in them had died out and they had therefore become souls and had been condemned to punishment in bodies, shall be anathema.
If anyone says or thinks that the soul of the Lord pre-existed and was united with God the Word before the Incarnation and Conception of the Virgin, let him be anathema.
If anyone says or thinks that the body of our Lord Jesus Christ was first formed in the womb of the holy Virgin and that afterwards there was united with it God the Word and the pre-existing soul, let him be anathema.
 John Paul II, Encyclical Fides et Ratio § 52
 2nd Council of Constantinople, Anathematisms of the Emperor Justinian against Origen, #1
 2nd Council of Constantinople, Anathematisms of the Emperor Justinian against Origen, #2
 2nd Council of Constantinople, Anathematisms of the Emperor Justinian against Origen, #3