Sadness, when defined, is the mental grief we feel because of our involuntary ailments—whether the evil be exterior, such as poverty, sickness or contempt; or interior, such as ignorance, dryness, depression or temptation.
Directly that the soul is conscious of some such trouble, it is downcast, and so trouble sets in.
Then we at once begin to try to get rid of it, and find means to shake it off; and so far rightly enough, for it is natural to us all to desire good, and shun that which we hold to be evil.
If anyone strives to be delivered from his troubles out of love of God, he will strive patiently, gently, humbly and calmly, looking for deliverance rather to God’s Goodness and Providence than to his own industry or efforts.
But if self-love is the prevailing object he will grow hot and eager in seeking relief, as though all depended more upon himself than upon God. I do not say that the person thinks so, but he acts eagerly as though he did think it.
Then if he does not find what he wants at once, he becomes exceedingly impatient and troubled, which does not mend matters, but on the contrary makes them worse, and so he gets into an unreasonable state of anxiety and distress, till he begins to fancy that there is no cure for his trouble.
Thus you see how a disturbance, which was right at the outset, begets anxiety, and anxiety goes on into an excessive distress, which is exceedingly dangerous.
[...] Just as internal commotions and seditions ruin a commonwealth, and make it incapable of resisting its foreign enemies, so if our heart be disturbed and anxious, it loses power to retain such graces as it has, as well as strength to resist the temptations of the Evil One, who is all the more ready to fish (according to an old proverb) in troubled waters.
Anxiety arises from an unregulated desire to be delivered from any pressing evil, or to obtain some hoped-for good. Nevertheless nothing tends so greatly to enhance the one or retard the other as over-eagerness and anxiety.
Birds that are captured in nets and snares become inextricably entangled therein, because they flutter and struggle so much.
Therefore, whensoever you urgently desire to be delivered from any evil, or to attain some good thing, strive above all else to keep a calm, restful spirit,—steady your judgment and will, and then go quietly and easily after your object, taking all fitting means to attain thereto.
Francis de Sales (1567-1622): Introduction to the Devout Life, 4, 11.