How could she be "full of Grace" if He weren't?
Mary is full of Grace because of what God did at her conception (the immaculate conception). The Angel Gabriel said this to her BEFORE she said YES to God. So, she was full of Grace, the Lord is with Thee. Obviously, everyone needs the Lord. Mary was a human being. She believed with her whole heart in God. She devoted her life to Him even before the Angel Gabriel came to her.
Then, the Angel tells Mary what God is going to do, and she is humble, reflecting how all humans should be. Even Jesus in His humanity was humble.
So it wasn't her saying YES that filled her with grace, she was full of grace and then said YES.
Luke 1:28-38Mary is the ultimate Christian: she said "be it done to me according to thy word", acknowledging that God is doing something great and wonderful to her, of which she has no ability to accomplish. Nothing is impossible with God. Why is the Immaculate Conception so difficult a concept to grasp? To me, it makes perfect sense. Mary is the Second Eve, and would be sinless like Eve prior to the Fall. God can preserve Mary from Original Sin and have Jesus, Son of God, Son of Man, born in a clean and pure "vessel". Jesus' humanity came from Mary alone, because there was no earthly father. If you can accept the Virgin Birth, why is the Immaculate Conception so difficult to accept?
28 And the angel being come in, said unto her: Hail, full of grace, the Lord is with thee: blessed art thou among women.29 Who having heard, was troubled at his saying and thought with herself what manner of salutation this should be.
30 And the angel said to her: Fear not, Mary, for thou hast found grace with God.
31 Behold thou shalt conceive in thy womb and shalt bring forth a son: and thou shalt call his name Jesus.
32 He shall be great and shall be called the Son of the Most High. And the Lord God shall give unto him the throne of David his father: and he shall reign in the house of Jacob for ever.
33 And of his kingdom there shall be no end.
34 And Mary said to the angel: How shall this be done, because I know not man?
35 And the angel answering, said to her: The Holy Ghost shall come upon thee and the power of the Most High shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God.
36 And behold thy cousin Elizabeth, she also hath conceived a son in her old age: and this is the sixth month with her that is called barren.
37 Because no word shall be impossible with God.
38 And Mary said: Behold the handmaid of the Lord: be it done to me according to thy word. And the angel departed from her.
Early Protestants had no problems with the Immaculate Conception or the Assumption of Mary. Early church fathers from the first, second, third centuries all acknowledged Mary's special role in our Salvation. I hope that you will think and pray on this.
God bless.