You're thinking in human, three dimensional terms. If that's the case, Satan was defeated at the cross and should no longer be tempting us. Further, we should no longer be dying, but living eternally, since Jesus conquered death on Cavalry, correct?
I submit that the Mass having been celebrated from the time of Jesus is proof that He was asking for a repeated memorial. The sacrifice, as such, happened once in our finite timeline, but in the infinite, God is witnessing His Son crucified this very day...and rising, too.
This is a mystery so beyond our miniscule brains, the notion that Jesus was intending a repeating memorial sacrifice on the altar is more than plausible. Time is our constraint, not God's. We are continually offering the body and blood of Christ in atonement for our sins.
His body is not ours to give.