Totally disagree. When I was a kid I also thought it was a song of depression, when I revisited the song recently (when Abish put it out recently) I realized it was hardcore Mussar.
Ok, whats the mussar message. - Lets analyze this, here are the lyrics:
שָׁקְעָה חַמָּה שָׁקְעָה נַפְשִׁי
בִּתְהוֹם רַעְיוֹנוֹת רַב כַּיָּם
כִּי נִלְאֵתִי מִלְּחֹם אֶת מִלְחַמְתִּי
עִם הַבָּשָׂר וְהַדָּם.
Here the composer laments his deeply depressing thoughts, and that he is wearied by the "war of the flesh"
This part has no positive message. Just despair.
יָמִים בָּאִים יָמִים חוֹלְפִים[2
מִבְּלִי קַחַת מִבְּלִי תֵּת
אִם לָזֹאת קָרָאתָ חַיִּים
אֱמֹר לִי אֵלִי מַה זֶּה מֵת.
This verse is undeniably negative - about a life that has no value and nothing to contribute or to gain. - No mussar school approves of this message. None.
מִי יוּכַל לִחְיוֹת כָּכָה
מִבְּלִי מֵשִׂים אֶת כָּל זֹאת
הַאִם לִשְׁכֹּחַ וְלִשְׂמֹחַ
אוֹ לִזְכֹּר וְלִבְכּוֹת
This is simply further expressing the how unbearable the depression is. - Where life only has value if one can forget the above thoughts of self pity and lack of self worth..
הַחֲיֵינִי אֵלִי עַד לְמָחָר
אוּלַי אֶפְתֹּר אֶת הַחֲלוֹם
שָׁקְעָה חַמָּה שָׁקְעָה נַפְשִׁי
עָבִים בָּאִים פָּנָה הַיּוֹם
Finally a prayer to hashem, to please help him deal with his sad thoughts of helplessness and usefulness. To give his life meaning.
Dont get me wrong. I love this song! This song is an amazing portrayal and expression of deep emotions. But it is not a "mussar" song at all.