You're right for saying our only mediator is Jesus -- this is true. Only one has been a mediator between man and God, and that's Christ -- this is the position of the Catholic Church as well, so the Church and you are in total agreement.
Mary isn't a secondary mediator between man and God, but we do intercede for one another when we pray for each other. Let's say Joe Schmoe is having problems with his spiritual life. He could go to his room and pray, one on one, to God, and I'd say that's not a bad thing at all--actually, it's a good thing. However, it would be even better if Joe went to his prayer group or to his church pastor and asked everyone to pray for him. The intercession of fellow Christians--especially ones close to God--is so much more powerful.
This is the role of Mary. We understand that no one has ever been so close to our Lord than she was because she is His mother. We also honor her (not worship her) because Jesus honored her since we are all commanded to honor our parents. When we honor her, we only mean to imitate Jesus.
So to briefly put it:
-Jesus is the only mediator between man and God.
-We have a duty, as Christians, to pray for each other and help direct each other to Christ.
-Mary, being the mother, was the closest person to Christ.
-Mary helps direct us to her son.
-We honor Mary as a mother because that's what Jesus did, and we want to be like Jesus.
That's the simplest way I could put it. I think without explanation, it does lead to a lot of misunderstanding.
What do you think about this passage? Didn't Jesus simply refute the same logic you are proposing here?
The True Family of Jesus
46 As Jesus was speaking to the crowd, his mother and brothers stood outside, asking to speak to him. 47 Someone told Jesus, “Your mother and your brothers are standing outside, and they want to speak to you.”
48 Jesus asked, “Who is my mother? Who are my brothers?” 49 Then he pointed to his disciples and said, “Look, these are my mother and brothers. 50 Anyone who does the will of my Father in heaven is my brother and sister and mother!”