On the contrary, such sadness is due to conceit,

וְאַדְּרַבָּה, הָעַצְבוּת הִיא מִגַּסּוּת הָרוּחַ,

for he does not know his place, and that is why he is distressed, because he has not attained the level of a tzaddik,

שֶׁאֵינוֹ מַכִּיר מְקוֹמוֹ, וְעַל כֵּן יֵרַע לְבָבוֹ עַל שֶׁאֵינוֹ בְּמַדְרֵגַת צַדִּיק,

to whom such foolish thoughts surely do not occur,

שֶׁלְּצַדִּיקִים בְּוַדַּאי אֵין נוֹפְלִים לָהֶם הִרְהוּרֵי שְׁטוּת כָּאֵלּוּ,

for were he to recognize his station, that he is very far from the rank of tzaddik,

כִּי אִילּוּ הָיָה מַכִּיר מְקוֹמוֹ, שֶׁהוּא רָחוֹק מְאֹד מִמַּדְרֵגַת צַדִּיק,

and would that he be a beinoni and not a rasha for even a single moment throughout his life (i.e., this is what he should be striving for at present rather than vainly desiring to be a tzaddik),

וְהַלְוַאי הָיָה בֵּינוֹנִי וְלֹא רָשָׁע כָּל יָמָיו, אֲפִילוּ שָׁעָה אַחַת,

then surely, this is the due measure of the beinonim and their task:

הֲרֵי זֹאת הִיא מִדַּת הַבֵּינוֹנִים וַעֲבוֹדָתָם –

to subdue the evil impulse and the thought that rises from the heart to the mind and to completely avert his mind from it, repulsing it as it were with both hands, as explained above in ch. 12.

לִכְבּוֹשׁ הַיֵּצֶר וְהַהִרְהוּר הָעוֹלֶה מֵהַלֵּב לַמּוֹחַ, וּלְהַסִּיחַ דַּעְתּוֹ לְגַמְרֵי מִמֶּנּוּ, וְלִדְחוֹתוֹ בִּשְׁתֵּי יָדַיִם, כַּנִּזְכָּר לְעֵיל.

The Alter Rebbe explained there that the evil in the soul of the beinoni remains vigorous; his task is to prevent it from expressing itself in thought, speech, and action. Thus, he has no control over the occurrence of evil thoughts in his mind, but only over his acceptance or rejection of these thoughts.