When an accusation stops the next mitzvah
Tzavaas HaRivash is a collection of teachings, rather than a will dictated by the Baal Shem Tov before his passing, as the Baal HaTanya explains in Iggeres HaKodesh 25. Section 44 describes a tactic of the yetzer hara. It can tell someone that he has committed a terrible transgression when the matter was only an extra stringency, or was not a transgression at all. Its aim is to leave him so dejected that he stops serving Hashem.
The rav connects this with Rebbe Nachman's opposition to taking on stringencies a person is not ready for, until the resulting heaviness undermines his avodah. Sichos HaRan 235 directly warns against excessive stringencies that lead to confusion and dejection. It also allows special care in a chosen mitzvah, while warning against unreasonable extremes even there.
The distinction between an obligation and a chumrah matters. A person should understand what is actually required, rather than treating every extra practice as if failure to keep it were the gravest sin. The answer is not to invent his own halachic standard. It is to refuse a distorted accusation that is being used to stop the next good act.
The teaching goes further: even where there has been a real fault, continuing to serve Hashem with joy can give Him greater nachas ruach than dwelling on the accusation until avodah stops. Its reasoning is that I am serving to give pleasure to my Creator, rather than making my own spiritual self-assessment the purpose of everything. Losing even the next moment of avodah to a destructive accusation defeats that purpose.
Tzavaas HaRivash 46 makes the qualification explicit: regret the wrongdoing, resolve not to repeat it, and return to joy in serving Hashem. This is a distinction between constructive regret and a dejection that cancels the rest of one's avodah.
The saying quoted about simcha and atzvus focuses on what each brings in its wake. Joy can lead toward mitzvos; dejection can expose a person to further failures. We are trying to protect the strength to serve. This does not declare every feeling of sadness a sin, or replace the discussion of a healthy broken heart and tears of joy.
Tzavaas HaRivash 45 itself distinguishes tears of joy. Likutey Moharan II:24 also gives a proper place to a broken heart at a suitable time. Those distinctions must remain alongside the strong warnings against atzvus.
Remembering Who is with me
Tzavaas HaRivash 137 gives an inward starting point for simcha: believe that the Shechinah is with you and guards you. You turn toward Hashem, and Hashem is looking toward you. Whatever the difficulty, the relationship has not disappeared.
The Baal HaTanya, Iggeres HaKodesh 11, develops this through the constant renewal of creation. Hashem gives a person existence at every moment. The present moment itself therefore carries a relationship with the Source of his life. The good may be hidden from his understanding, but he has not become independent of Hashem or abandoned by Him.
The letter describes the person who becomes dejected and complains as being like one who denies Hashem, because he is treating his life as though it were separate from its Giver and lacking the good that comes from Him. The rav presents this as the beginning of a loss of emunah: I imagine that Hashem has left me because of my sins or circumstances. Recognizing His constant presence answers that thought. The demand is strong, and the avodah is to examine my own awareness and strengthen my emunah. It is not permission to accuse another suffering person of lacking faith or to treat clinical depression as a religious verdict.
The verse gives words for the relationship within trouble:
יִקְרָאֵנִי ׀ וְֽאֶעֱנֵהוּ עִמּֽוֹ־אָנֹכִי בְצָרָה אֲחַלְּצֵהוּ וַאֲכַבְּדֵֽהוּ׃
He will call Me and I will answer him; I am with him in trouble. I will rescue him and honor him. (Tehillim 91:15)
The letters of besimcha and machshavah can be rearranged into one another. That association fits this work: directing thought toward Hashem's presence can help open a place for joy. The thought is something to return to, even while the difficulty remains.