As a former RCC, I would have no problem with the, say, Lutheran view of the Presence in the Eucharist. However, the idea that the Mass is a Sacrifice is clearly unscriptural. Jesus was sacrificed "once for all" as the book of Hebrews says many more times than "once for all".
As a former RCC(sic) you're ignorant of what the Mass is, which isn't uncommon for an overwhelming majority of poorly Catechized former Catholics.
He IS sacrificed "once for all". The offering made to God of the Body and Blood of His Son, Jesus, from the altar, is happening in Communion with the actual Crucifixion in the eyes of God. Eternity is timeless. What will come has already taken place in the eyes of God. What has gone is taking place this very second. In short, our sins today are contributing to the crucifixion that took place 2000 years ago in linear time. In God's eyes, it's still happening...