The Alter Rebbe distinguishes between atzvut (depression/sadness) — which he considers spiritually harmful and to be resisted — and merirut (bitterness of heart), a productive spiritual pain. He provides detailed guidance on managing low mood states without being paralyzed by them.
בְּרַם, כְּגוֹן דָּא צָרִיךְ לְאוֹדוֹעֵי כְּלָל גָּדוֹל: כִּי כְּמוֹ שֶׁנִּצָּחוֹן לְנַצֵּחַ דָּבָר גַּשְׁמִי, כְּגוֹן: שְׁנֵי אֲנָשִׁים הַמִּתְאַבְּקִים זֶה עִם זֶה לְהַפִּיל זֶה אֶת זֶה, הִנֵּה, אִם הָאֶחָד הוּא בְּעַצְלוּת וּכְבֵדוּת – יְנוּצַּח בְּקַל וְיִפּוֹל גַּם אִם הוּא גִּבּוֹר יוֹתֵר מֵחֲבֵירוֹ, כָּכָה מַמָּשׁ בְּנִצְחוֹן הַיֵּצֶר, אִי אֶפְשָׁר לְנַצְּחוֹ בְּעַצְלוּת וּכְבֵדוּת – הַנִּמְשָׁכוֹת מֵעַצְבוּת וְטִמְטוּם הַלֵּב כָּאֶבֶן, כִּי אִם, בִּזְרִיזוּת – הַנִּמְשֶׁכֶת מִשִּׂמְחָה וּפְתִיחַת הַלֵּב, וְטָהֳרָתוֹ מִכָּל נִדְנוּד דְּאָגָה וָעֶצֶב בָּעוֹלָם. אַךְ הָעַצְבוּת מִמִּילֵּי דִשְׁמַיָּא צָרִיךְ לָשִׁית עֵצוֹת בְּנַפְשׁוֹ לִיפָּטֵר מִמֶּנָּה, אֵין צָרִיךְ לוֹמַר בִּשְׁעַת עֲבוֹדָה, שֶׁצָּרִיךְ לַעֲבוֹד ה׳ בְּשִׂמְחָה וּבְטוּב לֵבָב, אֶלָּא אֲפִילוּ מִי שֶׁהוּא בַּעַל עֲסָקִים וְדֶרֶךְ אֶרֶץ, אִם נוֹפֵל לוֹ עֶצֶב וּדְאָגָה מִמִּילֵּי דִשְׁמַיָּא בִּשְׁעַת עֲסָקָיו, בְּיָדוּעַ שֶׁהוּא תַּחְבּוּלַת הַיֵּצֶר – כְּדֵי לְהַפִּילוֹ אַחַר כָּךְ בְּתַאֲווֹת חַס וְשָׁלוֹם, כַּנּוֹדָע, שֶׁאִם לֹא כֵן, מֵאַיִן בָּאָה לוֹ עַצְבוּת אֲמִיתִּית מֵחֲמַת אַהֲבַת ה׳ אוֹ יִרְאָתוֹ, בְּאֶמְצַע עֲסָקָיו. וְהִנֵּה, בֵּין שֶׁנָּפְלָה לוֹ הָעַצְבוּת בִּשְׁעַת עֲבוֹדָה בְּתַלְמוּד תּוֹרָה אוֹ בִּתְפִלָּה, וּבֵין שֶׁנָּפְלָה לוֹ שֶׁלֹּא בִּשְׁעַת עֲבוֹדָה, זֹאת יָשִׂים אֶל לִבּוֹ: כִּי אֵין הַזְּמַן גְּרָמָא כָּעֵת לְעַצְבוּת אֲמִיתִּית, אֲפִילוּ לְדַאֲגַת עֲוֹנוֹת חֲמוּרִים חַס וְשָׁלוֹם, רַק לָזֹאת צָרִיךְ קְבִיעוּת עִתִּים וּשְׁעַת הַכּוֹשֶׁר בְּיִשּׁוּב הַדַּעַת, לְהִתְבּוֹנֵן בִּגְדוּלַּת ה׳ אֲשֶׁר חָטָא לוֹ, כְּדֵי שֶׁעַל יְדֵי זֶה יִהְיֶה לִבּוֹ נִשְׁבָּר בֶּאֱמֶת בִּמְרִירוּת אֲמִיתִּית.
Truly this should be made known as a cardinal principle, that as with a victory over a physical obstacle, such as in the case of two individuals who are wrestling with each other, each striving to throw the other—if one is lazy and sluggish he will easily be defeated and thrown, even though he be stronger than the other, exactly so is it in the conquest of one’s evil nature; it is impossible to conquer it with laziness and heaviness, which originate in sadness and in a heart that is dulled like a stone, but rather with alacrity which derives from joy and from a heart that is free and cleansed from any trace of worry and sadness in the world. As for the sadness which is connected with heavenly matters, one must seek ways and means of freeing oneself from it, to say nothing of the time of Divine service, when one must serve G–d with gladness and a joyful heart. But even if he is a man of commerce and worldly affairs, should there enter into him any melancholy or anxiety about heavenly matters during the time of his business affairs, it is clearly a machination of evil impulse in order to lure him afterward into lusts, G–d forbid, as is known. For were it not so, whence would a genuine sadness, which is one that is derived from love or fear of G–d, come to him in the midst of his business affairs? Thus, whether the melancholy encroaches on him during Divine service, in study or prayer, or not during Divine service, he should tell himself that now is not the time for genuine anxiety, not even for worry over serious transgressions, G–d forbid. For, for this, one needs appointed times and a propitious occasion, with calmness of mind to reflect on the greatness of G–d, against Whom one has sinned, so that thereby one’s heart may truly be rent with sincere contrition.