Indeed, Jesus suffered and died but once, but as you say the effects of that sacrifice are perpetual. When I was a Protestant we would speak (and sing) of being “washed in the blood of the Lamb,” even though the Lamb physically shed His blood on one occasion millennia ago and halfway across the world. God still acts in our lives because of Jesus’ sacrifice.
Now, as the author of the letter to the Hebrews makes clear, Jesus does not offer repeated, ineffectual sacrifices for sin, nor does He continue to suffer His Passion in eternity. But He is still (not just “once was”) our High Priest before the Father, and even “seated” He continues His work of reconciling individual sinners with the Father (through the fruits of His one sacrifice) until it’s time to bring down the curtain on the present Creation.
In Catholicism, as you likely know, we believe that we are privileged to participate in Jesus’ offering of His singular sacrifice before the Father, through the Mass. The prophet Malachi speaks of the Gentiles throughout the world one day making a pure offering to the true God. The only pure and acceptable offering is God’s own spotless Lamb, and so we see that prophecy fulfilled in the Mass. Just as with the lambs and other animal sacrifices of the Old Covenant, first God provides the sacrificial subject (since we have nothing that does not come from Him) and then He graciously permits His People to offer it back to Him. In the case of Jesus, our involvement is even more intimate, since we are told the saved are incorporated into Christ and become members of His Body. Through that connection we are enabled to worship God with a fitting offering that we could never have made ourselves.
