Mary is the mother of Jesus. Jesus, God and The Holy Spirit is one and god I understand in the catholic sense. This is why Mary is the mother of God since Jesus is God, but this doesn't make sense in our everyday lives, so a Catholic would probably say we can never understand God or that God works in mysterious ways, some bull s*it like that. Remember this has been decided on by humans (Catholics) to be the "correct" interpretation of the scripture. I doesn't really mather, if God were to exist in accordance with the Catholic understand of scripture the correct answer would be that God have no father since God have existed for ever and will exist forever, otherwise you could always ask who is God's father's father, and his father before that?
The title "Mother of God" is not about God the Holy Trinty - but God the Son, Jesus Christ. The title "Mother of God" reflects the understand that Jesus is both fully human and fully divine. It came about at a time when people were denying the divinity of Christ - that Jesus is God. Calling Mary the "Mother of God" reinforces that Jesus is God the Son, part of the Holy Trinity.
As always it is impossible to be an ex-Christian. When true saving faith is supernatural created in the heart by the Holy Spirit it stays there through out eternity.
The Father of Jesus is God. The child created by the Holy Spirit, sustained by Maria. If you can believe God created Adam and Eve, it should not be difficult to believe He could create a human body supernatural inside Maria. Jesus spirit resided in that body, and this same body rose from death in a glorified version, that will last for eternity as the God-man Lord Jesus Christ.
a few theological corrections is needed. Jesus never said he was God. He did say in a prayer that He was "one" with God and invited us to become one with him. Jesus did say he was the Son of God. which means Mary was not the mother of God, she was the mother of the son of God who is/was foreordained to be OUR Saviour.
praying to Mother Mary, is not what Jesus taught us to do. Jesus taught us to pray to Heavenly Father in his name. so in one sense this could be a form of idolatry. another reason IMO not to be a catholic.
God the Father has been taught to be named Eloihem, and jesus is taught to be JWHW or jehovah. in english. clearly Eloihem is the Father of jesus. both are resurrected immortal personages. it sounds reasonable to me that Elohem has a Heavenly Father just like he has a son in Jesus. How many "generations" of the godhead are there, in the eternal sense of the matter, i do not know, but for right now the most important thing is, WE pray to our Heavenly Father and Jesus is the saviour/messiah.