Mary's "yes"

So yesterday was our parish's feast day, the Immaculate Conception. Are you the last Catholic to hear that this immaculate conception is the conception of MARY, not of Jesus? Well it's true! Rumor has it Mary was conceived (in her mother's womb- this is always specified. I guess it's to distract people from asking where she was conceived, which is too personal a question.) without sin. How about that?
Every year on this day I hear a sermon about Mary and how fortunate we are that this young woman said "yes" to God, and agreed to be the God-bearer, the first tabernacle, if you will. I ponder this. If it's true that she was chosen even before she was born, then what choice did she really have? Would it even be possible that she could have said no? How would that work? Did God have a couple of other girls conceived without sin, just as plan B,C,D? Or would her "no" have pushed back the date of the coming of our Messiah until God could work up another immaculate conception of some other girl and grow her up to child-bearing age?
But I guess she, being human like us, had a choice. Sure, she was specially gifted and prepared for this pivotal role in history, in the Kingdom. But ultimately, it came down to her accepting God's plan for her and agreeing to participate in it, using the gift God gave her. Did Mary know how special she was, how gifted she was for doing God's work? Maybe she had an inkling. How different would the world have been if she refused to move in the ways God asked her to move? Or, if she was too afraid to try?
I think the same is true for all of us. Maybe we have very little awareness of the gifts God has given us. Maybe we are afraid to put ourselves out there, afraid to trust that we have what we need within ourselves, put there by God. How different would the world be if we agree to move in the way that God is asking us to move?