The sacrifice took place 2000 years ago.
Yes, it did. That you (I assume) wrongly believe it ended 2000 years ago is irrelevant to the argument at hand though, because we don't believe that.
Therefore, according to our system of belief, we worship God by offering sacrifice. We honor Mary, but we do not offer sacrifices to her.
Yep ... and that one Sacrifice for all times and places is made present (praise God) at every Mass/Liturgy, every day, all over the world.
I am not asking you what you think about the Mass. I am asking you to correct the demonstrably wrong statement that there is no difference between the Catholic worship of God, which is the Holy Mass, and Catholic veneration of Mary, which is not.