I never tire of rational discussions relating to God and all His wonders.
You have touched on a very good description of the Mass. Christ’s sacrifice is eternal, it is outside of time. At the Mass we step out of time and into the etermal Sacrifice of Christ, it is always current in heaven, and is shared with us at Mass here on earth.
Since we are bound to time, something that does not exist in heaven, I sometimes describe the way the effects His sacrifice has on us like a ripple in a pond when a pebble is thrown in. In the center is Calvary, the ripples that go out are the effect travelling through time, forward and backward through time to reach us today, and those who lived before He was born on earth.
Now the description above is not 100% accurate, but sort of illustrates how the one Sacrifice of Calvary comes to us even today. It simplifies the issue to much, but it helps me as a starting point to understand how an event so long ago to us, can be present to us today.