Christ came "not that he might offer himself repeatedly..." but "once and for all". Christ came to offer His own blood "to take away sin by his sacrifice". What, then, are we to make of the sacrifice at Mass? Are we offering a sacrifice repeatedly when, as Jesus said on the cross, "it is finished"?
First, the word "Eucharist" means "thanksgiving", and we offer thanks to God "for all that he has accomplished through creation, redemption, and sanctification." 
It is the memorial that Jesus established when He told the apostles to "do this in memory of me".
Lastly - and to answer the question of one sacrifice versus many - the Eucharist represents the sacrifice at Calvary.  Read that carefully: it re-presents or presents again the sacrifice. We are made present again at Calvary as witnesses to Christ's singular offering of Himself for us.
I may not be in the room with you, but as you read these words, I am present in a way. If I call you on the phone, I am present to you though not physically in the same space. Similarly, we are present again (re-presented) at Calvary as Christ dies for us, giving us each the chance to claim that victory as our own.
Today, review the words spoken in the Liturgy of the Eucharist. You can listen to them as spoken at Mass, read them (at catholic-resources.org, or with commentary at sjy.org) or view a recorded Mass online. Meditate on the words spoken and find examples of thanksgiving, of memorial, and of our presence at Christ's sacrifice once and for all "so that sins may be forgiven".
Lord, by your cross and resurrection, you have set us free. You are the Savior of the World.