Many people are deterred from seriously attempting Christian chastity because they think (before trying) that it is impossible. But when a thing has to be attempted, one must never think about possibility or impossibility.
Faced with an optional question in an examination paper, one considers whether one can do it or not: faced with a compulsory question, one must do the best one can.
You may get some marks for a very imperfect answer: you will certainly get none for leaving the question alone.
Not only in examinations but in war, in mountain climbing, in learning to skate, or swim, or ride a bicycle, even in changing a flat tire – ask me about it -, people quite often do what seemed impossible before they did it.
It is wonderful what you can do when you have to.
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.