Hi Quantum,
I would like to ask you where you got this information. Catholic Christians do not ask Mary to forgive our sins.
Jesus Christ forgives sins (1 John 1:9) and He gave the Apostles and their successors the authority to forgive sins in His name (John 20:22-23).
If you are referring to the Rosary, it is a Prayer yes, but also a reflecting upon the Gospels. I posted the Biblical references of the Rosary here.
Hope this helps : )