Book IV, Chapter II of Introduction to the Devout Life (emph. mine):
“Anxiety of mind is not so much an abstract temptation, as the source whence various temptations arise. 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 any one 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.
“This unresting anxiety is the greatest evil which can happen to the soul, sin only excepted. 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. By easily I do not mean carelessly, but without eagerness, disquietude or anxiety; otherwise, so far from bringing about what you wish, you will hinder it, and add more and more to your perplexities. ‘My soul is alway in my hand, yet do I not forget Thy Law,’ David says. Examine yourself often, at least night and morning, as to whether your soul is ‘in your hand;’ or whether it has been wrested thence by any passionate or anxious emotion. See whether your soul is fully under control, or whether it has not in anywise escaped from beneath your hand, to plunge into some unruly love, hate, envy, lust, fear, vexation or joy. And if it has so strayed, before all else seek it out, and quietly bring it back to the Presence of God, once more placing all your hopes and affections under the direction of His Holy Will. Just as one who fears to lose some precious possession holds it tight in his hand, so, like King David, we ought to be able to say, ‘My soul is alway in my hand, and therefore I have not forgotten Thy Law.’
“Do not allow any wishes to disturb your mind under the pretext of their being trifling and unimportant; for if they gain the day, greater and weightier matters will find your heart more accessible to disturbance. When you are conscious that you are growing anxious, commend yourself to God, and resolve steadfastly not to take any steps whatever to obtain the result you desire, until your disturbed state of mind is altogether quieted;—unless indeed it should be necessary to do something without delay, in which case you must restrain the rush of inclination, moderating it, as far as possible, so as to act rather from reason than impulse.
“If you can lay your anxiety before your spiritual guide, or at least before some trusty and devout friend, you may be sure that you will find great solace. The heart finds relief in telling its troubles to another, just as the body when suffering from persistent fever finds relief from bleeding. It is the best of remedies, and therefore it was that St. Louis counseled his son, ‘If thou hast any uneasiness lying heavy on thy heart, tell it forthwith to thy confessor, or to some other pious person, and the comfort he will give will enable thee to bear it easily.'”