ת"ר לפי שראה אדם הראשון יום שמתמעט והולך אמר אוי לי שמא בשביל שסרחתי עולם חשוך בעדי וחוזר לתוהו ובוהו וזו היא מיתה שנקנסה עלי מן השמים עמד וישב ח' ימים בתענית [ובתפלה]
With regard to the dates of these festivals, the Sages taught: When Adam the first man saw that the day was progressively diminishing, as the days become shorter from the autumnal equinox until the winter solstice, he did not yet know that this is a normal phenomenon, and therefore he said: Woe is me; perhaps because I sinned the world is becoming dark around me and will ultimately return to the primordial state of chaos and disorder. And this is the death that was sentenced upon me from Heaven, as it is written: “And to dust shall you return” (Genesis 3:19). He arose and spent eight days in fasting and in prayer.
״אַל תֵּצַר צָרַת מָחָר, כִּי לֹא תֵדַע מַה יֵּלֶד יוֹם. שֶׁמָּא מָחָר בָּא וְאֵינֶנּוּ, נִמְצָא מִצְטַעֵר עַל הָעוֹלָם שֶׁאֵין שֶׁלּוֹ״. ״מְנַע רַבִּים מִתּוֹךְ בֵּיתֶךָ, וְלֹא הַכֹּל תָּבִיא בֵּיתֶךָ״. ״רַבִּים יִהְיוּ דּוֹרְשֵׁי שְׁלוֹמֶךָ. גַּלֵּה סוֹד לְאֶחָד מֵאָלֶף״.
The Gemara quotes additional statements from the book of Ben Sira: Do not suffer from tomorrow’s trouble, that is, do not worry about problems that might arise in the future, as you do not know what a day will bring. Perhaps when tomorrow comes, the individual who was so worried will not be among the living, and he was consequently upset over a world that is not his. Prevent a crowd from inside your house, do not let many people enter, and do not even bring all your friends into your house. Make sure, however, that a crowd seeks your welfare, and that you have many allies. Reveal a secret to only one in a thousand, since most people are unable to keep a secret.
״דְּאָגָה בְלֶב אִישׁ יַשְׁחֶנָּה״, רַבִּי אַמֵּי וְרַבִּי אַסִּי חַד אָמַר: יַשִּׂחֶנָּה מִדַּעְתּוֹ, וְחַד אָמַר: יְשִׂיחֶנָּה לַאֲחֵרִים.
In a similar vein, one is urged to relieve his distress. The verse states: “If there is a care in the heart of a man, let him bend it [yashḥena]” (Proverbs 12:25). Rabbi Ami and Rabbi Asi dispute the verse’s meaning. One said: He should force it [yasḥena] out of his mind. He should remove his worries from his thoughts. And one said: It means he should tell [yesiḥena] his troubles to others, which will relieve his anxiety.
״דְּאָגָה בְלֶב אִישׁ יַשְׁחֶנָּה״, רַבִּי אַמֵּי וְרַבִּי אַסִּי, חַד אָמַר: יַשִּׂחֶנָּה מִדַּעְתּוֹ, וְחַד אָמַר: יְשִׂיחֶנָּה לַאֲחֵרִים.
§ The Gemara explains another verse in Proverbs: “If there is care in a man’s heart, let him quash it [yashḥena]” (Proverbs 12:25). Rabbi Ami and Rabbi Asi dispute the verse’s meaning. One said: He should forcefully push it [yasḥena] out of his mind. One who worries should banish his concerns from his thoughts. And one said: It means he should tell [yesiḥena] others his concerns, which will lower his anxiety.
דְּאָגָ֣ה בְלֶב־אִ֣ישׁ יַשְׁחֶ֑נָּה וְדָבָ֖ר ט֣וֹב יְשַׂמְּחֶֽנָּה׃
If there is anxiety in a man’s mind let him quash it,And turn it into joy with a good word.
Worry in a person's heart - When it occurs that worry is in a person's heart, he needs to strive to subdue and lower the worry, for in worry, there is no use at all, only harm. And this is not worthy for an enlightened person, for the worry about a thing that will pass is craziness. Also the worry about a future matter is worthless, because it is worthwhile for the person to trust in HaShem Yitbarach (God, who is blessed).
(יא) שלשה מביאין לידי דאגה. צער גדול. וגדול צפרנים. ובגדים פצועים:
Three things bring anxiety: great pain, long fingernails, and ripped clothing.
אַשְׁרֵ֣י אָ֭דָם מְפַחֵ֣ד תָּמִ֑יד וּמַקְשֶׁ֥ה לִ֝בּ֗וֹ יִפּ֥וֹל בְּרָעָֽה׃
Happy is the man who is anxious always,But he who hardens his heart falls into misfortune.
R' Schneur Zalman of Liady, Tanya, Likkutei Amarim, chap. 26 (circa 1800)translated by R' Norman Lamm, The Religious Thought of Hasidism
I offer this advice on how to purge one's heart of all anxiety and of every trace of worry in mundane matters, even about 'children, health, and sustenance.' Everyone knows the dictum of the Rabbis that 'just as one must recite a blessing for the good [so must one recite a blessing for misfortune.'] (Mishnah Brachot 9:5) The Gemara explains that one should accept bad news with the same joy with which he would greet manifest and obvious good news, 'for this too is for the good' [gam zu l'tovah] (Brachot 60b.), except that it is not apparent and evident to human eyes [...]
As for the sadness in regard to spiritual matters [i.e., the anxiety that comes about as a result of moral and religious introspection and self-criticism], one must devise ways to rid oneself of it, and most certainly so at the time of divine worship, when one must serve God with joy and a glad heart. [...]
Hence, whether the melancholy seizes him during divine service -- Torah study or prayer -- or it befalls him at some other time, he should persuade himself that now is not the time for genuine sadness, not even for worry over serious transgressions, God forbid. For this, one needs set times and the right occasion to reflect with a calm mind on the greatness of God, against whom he has sinned, so that thereby one's heart may truly be rent with sincere contrition. It is explained elsewhere when this time should be, and, further, that as soon as his heart has been broken during these specific times, he should completely remove the anxiety from his heart and believe with a perfect faith that the Lord has dismissed his sin and is abundantly forgiving. This is the true joy in God which comes after anxiety, as mentioned above.
"...The most propitious time, specifically appropriate for most people, is when one is in any case troubled by mundane worries or dejection, without apparent cause. Then is the right time to transform the sadness by becoming one of those 'masters of self-examination' mentioned earlier... thus ridding oneself of the dejection occasioned by mundane affairs." - ibid., Chapter 31