You confuse the sacrifice with the partaking of Communion. It is the sacrifice upon the altar from which the graces of Christ's Crucifixion flow into the world. These graces are what heals the world and brings about our salvation. They all come from the Cross, through the Eucharist into the world.
So it doesn't say, as you believe, that taking communion forgives sin. It says that the action of the Eucharist on the altar, the giving of oblation, the One Perfect Oblation to the Father provides blessings into the world, provides grace for all. And this grace leads us into the patterns of behavior, strengthens us to live without sin. And sources the other sacraments, like Confession.
I don't expect you to understand this larger idea, I've given up on that a long time ago. But it would not be to much to ask that you undertand at least that there is somethign here larger than your abilty to understand it.
Of course, you will just say "semantics" or some such ploy in order to avoid acknowledging that we believe something more complex than you can conceive of.
SD