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 a long 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 towards 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.1This section of the book contains a Lewis sentence that is often quoted out of context: "The only fatal thing is to sit down content with anything less than perfection." When taken by itself it has the potential for inducing guilt; but, understood in its context it is about training us in the habits of our souls. It is about wanting to act a certain way because Jesus, who died for our sins, is the one we choose to obey. Thus, we seek nothing less than perfection recognizing that when we fail, and we will fail, we have forgiveness.
1. Lewis, C.S. Mere Christianity. Glasgow: William Collins Sons and Co. Ltd., 1978, p. 91.