"If only Jupiter would grant me back the years that have passed!"

Here's a breakdown:

* O mihi: "Oh, for me"

* praeteritos: "past"

* referat: "bring back, grant"

* si Jupiter annos: "if Jupiter would [grant me] the years"

The speaker, Aeneas, is longing for the past, particularly the time before the Trojan War and the destruction of Troy. He wishes he could relive those happier days.