Mary is the mother of Jesus Christ, God-made-man. That does not mean that she existed before the Creator of heaven and earth. She adored him as his humble servant. She said to the angel: "Behold, I am the handmaid of the Lord", Yet the second Person of the Blessed Trinity, God the Son, became a member of the human race to give his life for humankind.
A mother brings forth a person, not just a human body or a human nature. It was a person who lay in Mary's womb, was fed at her breast, lay in the manger, and was carried in her arms. Since that person was God-made-man, she is truly the Mother of God.
All her gifts and privileges were given precisely because she was God's mother. No other human being has ever been called upon to fulfil such an exalted office, and no other human being has been so honoured and blessed by God. Hence she could humbly say, "Behold, henceforth all generations will call me blessed".