Starting with a definition of grace
Our justification comes from the grace of God. Grace is favor, the
free and undeserved help that God gives us to respond to his call to
become children of God, adoptive sons, partakers of the divine
nature and of eternal life. [cf. Jn 1:12-18; 17:3; Rom 8:14-17; 2
Followed by a definition of prayer
What is prayer?
Prayer is the raising up of the mind and heart to God.
Understanding the question, if grace is from God, why Mary?
A. Because God, in his goodness has wanted his creatures, as his instruments, his servants, and ultimately as his children, to participate in his works, which include those of grace. Mary case is unique, she is not the source of grace, but the Mediatrix of all graces, i.e., through whom all graces are distributed and come to us. Easy to understand because the source of grace, Jesus, came through her.
I watched My Big Fat Greek Wedding awhile back and if I recall correctly, the mother was explaining to the bride to be the relationship in a home, between husband and wife, how the wife is like the neck, pointing the head to where it should look. Borrowing, we can think of Mary mediatrix as who through whom all that flows to the [Mystical] body and from the body to the head[=Christ], passes.
Prefacing before answering the other question posed, I did not follow the logic that concludes that we are full of grace, because, we are not and we need only look at ourselves, and at the world around us.
If God gives it freely, why do we need to pray?
A. Praying being raising our mind (thinking of him) and heart (loving him) to God, talking with him, is what a good child does with a Father who madly loves them.
Praying for grace, he has it, we do not. It is what a needy child does before his [almighty] Father, who cannot deny him anything that is good for them.[cf. Mt 7:11]
cf. Lk 11:13
13 If you then, who are evil, know how to give good gifts to your
children, how much more will the heavenly Father give the Holy
Spirit to those who ask him!”
cf. II. Grace | Catechism of the Catholic Church, 1996-2005 esp. 2003
Closing note: A catholic ought to be obedient and submit to the Church's judgment on the supposed Marian apparitions at Medjugorje.