Mary knew that she was the vessel of the Holy one. The angel Gabriel told her. Did she know that her son would preform miracles? Did she know he would heal the sick and comfort the broken? Did she know he would suffer and give his life for humanity?
I believe she did not know the extent to which Jesus would live out his mission. Yet, she loved him the way only a mother can and like all mothers, she let her child go to fulfill his destiny.
What Mary knew was that she would and did follow God’s will for her life and she knew God loved her. That is all she needed to know.