If grace cannot do that, the very first step, it cannot do other things either. Then the person is blind. He is eternally lost unless someone would put an extraordinary weight into the scales, as it were, to call for an extraordinary grace. Such a grace is comparable to a miracle, and it can forestall or even cancel out human resistance. Suffering is also needed for reparation for sin.
Every sin is a debt, which unbalances the scale of the objective order.
The holiness of God loves all that is good, and so wants it rebalanced. Suffering freely accepted will rebalance. He was not obliged - the only means was the incarnation of a Divine Person. He did that. We can have merit, a claim to reward, in that we become members of the Incarnate Divine Person, Christ, and in as much as we are like Him - like Him in suffering for reparation.