There are primary sources for Greco-Roman myth stretching all the way back to the 8th century BCE, with some Near-Eastern precursors stretching back even further, into the Bronze Age. You are right that during the Renaissance, there was a reawakening of interest in ancient myth, and thus an urgent search for lost or endangered manuscripts of ancient writing, but there was no fabrication, at least not of ancient myth. Men like John Keats wrote works inspired by the myths, like his Hyperion, but these were not passed off as ancient texts, nor would they have been able to do so.

Hesiod's didactic poems Theogony and Works and Days are important early sources (7th century BCE). They survive to us in many manuscripts and papyri, fully or in fragments. It is a similar situation for Homer, another important source of early myth. 700 years later, the Roman poet Ovid performed an important collation in his massive Methamorphoses. He preserves many more obscure myths which might have been lost otherwise, and Ovid manuscripts were preserved throughout antiquity, through the middle ages, and into the Renaissance. The Imperial Roman author Hyginus helped preserve material as well. Skipping forward another 1300 years, we have a collection of texts by anonymous authors of unknown date called "The Vatican Mythographers" who collected sanitized versions of ancient myth. So in short, people never stopped studying myth.