How do Catholic scholars explain Gal 2: 21 against the backdrop of Mtt 5: 17-18?

We hear St Paul telling in Gal 2:21 (NRSVCE): > I do not nullify the grace of God; for if justification comes through the law, then Christ died for nothing. But we see Jesus telling in Mtt 5: 17-18 : > Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil. For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. One finds some mismatch between Mtt 5: 17-18 and Gal 2:21 in so for as the role of law in salvation is concerned. If Jesus came to fulfil the law , why does St Paul downplay its role ? Moreover, how are those who lived before Jesus's redemptive mission, but were faithful to the law, justified ? My question therefore is: How do Catholic scholars explain Gal 2: 21 against the backdrop of Mtt 5: 17-18 ?