One of the best explanations I've ever heard about the eternal sacrifice of the Eucharist actually comes from a Protestant, the Swiss theologian Karl Barth. He is stated as saying, "The crucifixion is an eternal event which occurred within history."
This is exactly what the Eucharist is. When we enter into the Divine Liturgy, we enter mystically into the heavenly temple, into eternity. We are joined by the heavens in proclaiming Christ as God, and partake continually in the once-for-all sacrifice of the Cross. Christ is only sacrificed historically once, but we enter into that self-same sacrifice eternally, at each Liturgy in the Heavenly Kingdom.