According to the Catechism:
II. . . .BORN OF THE VIRGIN MARY
487 What the Catholic faith believes about Mary is based on what it believes about Christ, and what it teaches about Mary illumines in turn its faith in Christ.
Mary’s predestination
488 “God sent forth his Son”, but to prepare a body for him, he wanted the free co-operation of a creature. For this, from all eternity God chose for the mother of his Son a daughter of Israel, a young Jewish woman of Nazareth in Galilee, “a virgin betrothed to a man whose name was Joseph, of the house of David; and the virgin’s name was Mary”:
The Father of mercies willed that the Incarnation should be preceded by assent on the part of the predestined mother, so that just as a woman had a share in the coming of death, so also should a woman contribute to the coming of life.
489 Throughout the Old Covenant the mission of many holy women prepared for that of Mary. At the very beginning there was Eve; despite her disobedience, she receives the promise of a posterity that will be victorious over the evil one, as well as the promise that she will be the mother of all the living. By virtue of this promise, Sarah conceives a son in spite of her old age. Against all human expectation God chooses those who were considered powerless and weak to show forth his faithfulness to his promises: Hannah, the mother of Samuel; Deborah; Ruth; Judith and Esther; and many other women. Mary “stands out among the poor and humble of the Lord, who confidently hope for and receive salvation from him. After a long period of waiting the times are fulfilled in her, the exalted Daughter of Sion, and the new plan of salvation is established.”
The Immaculate Conception
490 To become the mother of the Savior, Mary “was enriched by God with gifts appropriate to such a role.” The angel Gabriel at the moment of the annunciation salutes her as “full of grace”. In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God’s grace.
491 Through the centuries the Church has become ever more aware that Mary, “full of grace” through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854:
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin.
492 The “splendor of an entirely unique holiness” by which Mary is “enriched from the first instant of her conception” comes wholly from Christ: she is “redeemed, in a more exalted fashion, by reason of the merits of her Son”. The Father blessed Mary more than any other created person “in Christ with every spiritual blessing in the heavenly places” and chose her “in Christ before the foundation of the world, to be holy and blameless before him in love”.
493 The Fathers of the Eastern tradition call the Mother of God “the All-Holy” (Panagia), and celebrate her as “free from any stain of sin, as though fashioned by the Holy Spirit and formed as a new creature”. By the grace of God Mary remained free of every personal sin her whole life long.
“Let it be done to me according to your word. . .”
494 At the announcement that she would give birth to “the Son of the Most High” without knowing man, by the power of the Holy Spirit, Mary responded with the obedience of faith, certain that “with God nothing will be impossible”: “Behold, I am the handmaid of the Lord; let it be [done] to me according to your word.” Thus, giving her consent to God’s word, Mary becomes the mother of Jesus. Espousing the divine will for salvation wholeheartedly, without a single sin to restrain her, she gave herself entirely to the person and to the work of her Son; she did so in order to serve the mystery of redemption with him and dependent on him, by God’s grace:
As St. Irenaeus says, “Being obedient she became the cause of salvation for herself and for the whole human race.” Hence not a few of the early Fathers gladly assert. . .: “The knot of Eve’s disobedience was untied by Mary’s obedience: what the virgin Eve bound through her disbelief, Mary loosened by her faith.” Comparing her with Eve, they call Mary “the Mother of the living” and frequently claim: “Death through Eve, life through Mary.”
And as previous posters have already said several times God exists outside of time. He knows all; past, present, and future. This does not restrict free will. We still freely choose, but God knows what we will choose because he is not restricted by time (obviously, since he created time itself). Much has been written about God and free will, I suggest you ask your priest to explain this more to you and perhaps suggest some essays or books that cover the topic at greater length. In the end though you must have faith.
