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 for 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.