The following is from Introduction to the Devout Life, the classic spiritual book written by our beloved saint of the say.
"Anxiety is not only a temptation, but a source from which and by which many temptations arise. I will therefore say something concerning it. Sadness is nothing else but that grief of mind which we have over an evil that we experience against our will, whether it be exterior, like poverty, sickness, or contempt, or interior like ignorance aridity discontent, or temptation. When the souls perceives that some evil has fallen her, she is displeased at having it and hence sadness follows. Immediately she desires to be free of it and to have a way of ridding herself o fit. Thus far she is right, for everyone naturally desires to embrace good and flies from what he thins to be evil.
"If out of love for God the soul seeks a way to be freed from her troubles, she will seek it with patience, meekness, humility, and tranquility. She will expect deliverance more from the providence of God than from her own labor, industry, or diligence. If she seeks her deliverance from a motive of self-love, then will she excite and fatigue herself in search of this means, as if the success depended more on herself than on God. I do not say that she thinks so, but that she acts as if she thought so. Now, if she does not succeed immediately according to her wishes, she falls into great anxiety and impatience. Instead of removing the evil, she increases it. It involves her in an excessive anguish and distress, with so great a loss of courage and strength that she imagines her evil incurable. You see, the, that sadness, which is justified in the beginning produces anxiety. Anxiety again produces an increase of sadness, and this is extremely dangerous.
Anxiety is the greatest evil that can befall the soul, sin only excepted. The seditious and intestine troubles of a commonwealth ruin it completely and prevent it from being able to resist a foreign invasion So, also, when our heart is troubled and disturbed within itself, it loses the strength necessary to maintain the virtues it had acquired. At the same time it loses the means to resist the temptations of the enemy, who then uses his utmost efforts to fish, as they say, in troubled waters.
Anxiety proceeds from an inordinate desire of being delivered from the evil that we feel or of acquiring the good that we hope for. Yet there is nothing that tends more to increase evil and to prevent the enjoyment of good than inquietude an anxiety. Birds remain caught in nets and traps because when they find themselves ensnared, they eagerly flutter about and struggle to extricate themselves and in that way entangle themselves all the more. Whenever you are pressed with a desire to be freed from some evil or to obtain some good, before all else be careful both to settle your mind in repose and tranquility and to compose your judgement and will. Then gently and meekly procure the accomplishment of your desire taking in regular order the means that may be most convenient. When I say gently, I do not mean carelessly but without hurry, trouble or anxiety. Otherwise, instead of obtaining the effect you desire, you will mar all and embarrass yourself the more.
'My soul is continually in my hands, O Lord, and I have not forgotten Thy law,' said David. Examine frequently during the day, or at least in the morning and evening, whether you have your soul in your hands, or whether some passion or anxiety has not robbed you of it. Consider whether yo have your heart at your command, or whether it has not escaped out of your hands to engage itself to some disorderly affection of love, hatred, envy, covetousness, fear, uneasiness, or joy. If it has gone astray, seek after it before you do anything else and bring it back quietly into the presence of God, subjecting all your affections and desires to the obedience and directions of His divine will. They who are afraid of losing anything which is precious hold it fast in their hands. So also, in imitation of this great kind, we should always say, 'O my God! my soul is in danger, and therefore I carry it always in my hands; and in this manner I have not forgotten Thy holy law."
Do not permit your desires, however small or trivial they may be, to to disquiet you. For after the little ones, those which are grater and more important may find your heart involved in trouble and disorder. When you perceive that anxiety begins to affect your mind, recommend yourself to God. Resolve to do nothing that your desire demands until it is restored to tranquility, unless it should be something that cannot be deferred. In that case, you must with a meek and tranquil effort hold back the current of your desires, restraining and moderating them as much as possible. Where upon, perform the action, not according to your desire, but according to reason.
If you can disclose the cause of your anxiety to your spiritual director, or at least to some faithful and devout friend, be assured that you will quickly find ease... Saint Louis the king gave this counsel to his son: 'If thou hast any uneasiness in thy heart, tell it immediately to thy confessor or to some good person. Thus thou shalt be enabled to bear thy evil very easily by the comfort he will give thee."