111 Yet in order that the grace of this sacrament may produce its full fruit, there is need, as we have already pointed out, of the cooperation of the married parties; which consists in their striving to fulfill their duties to the best of their ability and with unwearied effort. For just as in the natural order men must apply the powers given them by God with their own toil and diligence that these may exercise their full vigor, failing which, no profit is gained, so also men must diligently and unceasingly use the powers given them by the grace which is laid up in the soul by this sacrament. Let not, then, those who are joined in matrimony neglect the grace of the sacrament which is in them; for, in applying themselves to the careful observance, however laborious, of their duties they will find the power of that grace becoming more effectual as time goes on. And if ever they should feel themselves to be overburdened by the hardships of their condition of life, let them not lose courage, but rather let them regard in some measure as addressed to them that which St. Paul the Apostle wrote to his beloved disciple Timothy regarding the sacrament of holy Orders when the disciple was dejected through hardship and insults: "I admonish thee that thou stir up the grace which is in thee by the imposition of my hands. For God hath not given us the spirit of fear; but of power, and of love, and of sobriety."
Source: Casti Connubii (Vatican.va)