It will help if you understand what the Church means by such doctrines and what it does not mean. First, the Church recognizes that Jesus in the ultimate sense is our only redeemer—plain and simple. Only God could make up for an offense against his divinity. When Jesus, the second Person of the Blessed Trinity, became man, he used the services of several human beings. he used prophets, the last of whom was his cousin, John the Baptist. He used St. Joseph as his foster father to protect him and to be a father to him in his formative years. Most of all, he used Mary as his mother who gave birth to him, nursed him, and nurtured him as a child. All of these people cooperated with him and his mission of salvation. He alone was the redeemer in the ultimate sense, but they cooperated with him in his work of redemption. In varying degrees they all could be called co-redeemers because of such cooperation. But because of her unique role and the degree of her cooperation, Mary is singled out. In all of humanity, God singled her out for a truly sublime role. Nursing Almighty God at her breast is beyond our ability to fully appreciate. Yet thousands of Christians since the Protestant reformation have completely ignored such sublimity.
What is said of co-redemptrix is also true of mediatrix. Because these terms can be highly misleading, the Church has not infallibly proclaimed them. (See article: www.ewtn.com/library/MARY/ORMARIA2.HTM) Nevertheless, God is the one who singled Mary out for the unique role in salvation that she has. She did not seek out such distinction. It is important to remember the high praise Jesus lavished on St. John the Baptist. Yet his mission was not nearly as exalted as Mary’s. Jesus worked his first miracle at her request. All she needed to say was: "They have no wine." He understood exactly what she wanted. He could have taken care of the matter on his own. But he chose to have his mother’s intercession be a part of the mix. The miracle wasn’t any less significant because of her part in it. On the contrary, she shows us how accessible he is to our needs. To truly appreciate Mary is to appreciate her Son all the more.