The doctrine of the Assumption means that Mary’s body was taken to heaven (whether this happened while she was alive or after she died isn’t specified) and that she now lives in her resurrected body. She is in heaven with God, as we all hope to be eventually. She isn’t God, isn’t part of the Trinity, etc., etc.
St. Therese of Lisieux said she would spend her heaven doing good on earth. Apparently God has permitted this, since many people who pray for her intercession are helped. Surely, Jesus would allow this pleasure to his mother. As we are all part of Christ’s body, and Mary is his mother, so she is also the mother of the church. She was with the apostles and took an interest in them. It makes perfect sense to me, that if I turn to her as my mother, she will help me with her prayers and that God would permit her even to help me more directly.
I think of when I was little and my brother had a birthday. My mother would help me make a present for him or go with me to buy something for him. She knew it would make me happy to give it to him. I think that sometimes God lets Mary and the saints help us because they like to do it.
But we don’t pray to Mary or the saints instead of praying to God or because we are afraid to approach God. Our ultimate goal is union with God; we only have the communion of saints because we are all united to Him.