The following words from C.S. Lewis are specifically about the Christian virtue of chastity, which is the moral teaching that the joy and responsibility of sex is for marriage only. Lewis acknowledges, that such teaching is contrary to our sexual instincts, and in Mere Christianity, he provides a masterful defense of why chastity, even in light of our strong sexual proclivities, is the only real path for full human flourishing. Beyond the specificity of chastity, however, I believe that Lewis’ words, as he indicates, are true of any virtue a Christian is attempting to develop. And so, I share it with you.
We may, indeed, be sure that perfect chastity–like perfect charity–will not be attained by any merely human efforts. You must ask for God’s help. Even when you have done so, it may seem to you for along time that no help, or less help than you need, is being given. Never mind. After each failure, ask forgiveness, pick yourself up, and try again. Very often what God first helps us toward is not the virtue itself but just this power of always trying again. For however important chastity (or courage, or truthfulness, or any other virtue) may be, this process trains us in habits of the soul which are more important still. It cures our illusions about ourselves and teaches us to depend on God. We learn, on the one hand that we cannot trust ourselves even in our best moments, and on the other, that we need not despair even in our worst, for our failures are forgiven. The only fatal thing is to sit down content with anything less than perfection.
By the way, in case anyone is tempted to think that married couples are released from the struggles of chastity, Christian teaching requires couples to be completely faithful, body and soul, which is to say that even their imaginations are to be pure in their devotion to one another. The point of chastity, and all the virtues for that matter, is that God desires his children to be completely free from the need to find fulfillment in anything or anyone other than himself, so that in such freedom his children may experience the fullness of joy, and become the embodiment of his glory.