How can Mary be God's Mother, but not the Mother of the Trinity?
Saying that Mary is the Mother of God sounds like saying she is the source of Jesus' divinity and the Mother of the Trinity. How should I understand this?
Understood correctly, the title Mother of God (Greek: Theotokos, "God-bearer") does not mean that Mary is the source of Christ’s divine nature, nor does it mean Mary is the Mother of the Father or the Holy Spirit. It means Mary is Mother of the person of Jesus, who is God:
The One whom [Mary] conceived as man by the Holy Spirit, who truly became her Son according to the flesh, was none other than the Father’s eternal Son, the second person of the Holy Trinity. Hence the Church confesses that Mary is truly "Mother of God." (CCC 495)