[Actually I do have an idea of where the self esteem thing came from [Eric Fromm], but I am not sure if that was the original source. I seem to recall some previous source. Nietzsche? I just can't put my finger on it this second.]
Nietzsche: Das Kriterium der Wahrheit liegt in der Steigerung des Machtgefühls. "The criterion of truth resides in the heightening of the feeling of power." The psychologists picked it up from him. That is not rare. Most of their ideas come from Nietzsche.
Later I started noticing even Musar books that were distorting the message of Musar and insisting just like the psychologists that self esteem was a good thing.
Recently more serious psychologists have noticed that they were mistaken and rather that self esteem being a good thing it is the cause of violence and evil.
So Musar really had this right from the beginning. This shows that the idea of Israel Salanter about the importance of learning Musar is really correct.
Appendix: Each one of the major Musar books talks about the evil of Pride with no exception. חובות לבבות אורחות צדיקים מסילת ישרים שערי תשובה מעלות המידות ספר היראה המיוחס לרבינו תם.
And furthermore all the disciples of Israel Salanter said the same thing. Isaac Blazzer, Yoseph Yozel Horvitz, etc.
The self esteem thing as the of cause of evil