What is Mussar? Why is it needed?
The Jewish Mission is to live according to the Torah of HaShem. This is summarized in the following verses:
Judaism is a way of life according to the Torah
Living a life of Judaism requires learning (we need to know what to do) and action (we need to do it). The question is which one is greater?
In connection to the mishna’s statement about the importance of Torah study, the Gemara relates the following incident: And there already was an incident in which Rabbi Tarfon and the Elders were reclining in the loft of the house of Nit’za in Lod, when this question was asked of them: Is study greater or is action greater? Rabbi Tarfon answered and said: Action is greater. Rabbi Akiva answered and said: Study is greater. Everyone answered and said: Study is greater, but not as an independent value; rather, it is greater as study leads to action.
Learning and doing both necessary. There is no place for learning without the intention to fulfill.
מַרְגְּלָא בְּפוּמֵּיהּ דְּרָבָא: תַּכְלִית חָכְמָה — תְּשׁוּבָה וּמַעֲשִׂים טוֹבִים, שֶׁלֹּא יְהֵא אָדָם קוֹרֵא וְשׁוֹנֶה וּבוֹעֵט בְּאָבִיו וּבְאִמּוֹ וּבְרַבּוֹ וּבְמִי שֶׁהוּא גָּדוֹל מִמֶּנּוּ בְּחָכְמָה וּבְמִנְיָן, שֶׁנֶּאֱמַר: ״רֵאשִׁית חָכְמָה יִרְאַת ה׳ שֵׂכֶל טוֹב לְכָל עוֹשֵׂיהֶם״. ״לָעוֹשִׂים״ לֹא נֶאֱמַר, אֶלָּא ״לְעוֹשֵׂיהֶם״ — לָעוֹשִׂים לִשְׁמָהּ וְלֹא לָעוֹשִׂים שֶׁלֹּא לִשְׁמָהּ. וְכָל הָעוֹשֶׂה שֶׁלֹּא לִשְׁמָהּ, נוֹחַ לוֹ שֶׁלֹּא נִבְרָא.
The objective of Torah wisdom is to achieve repentance and good deeds;
that one should not read the Torah and study mishna and become arrogant
and spurn his father and his mother and his teacher
and one who is greater than he in wisdom or in the number of students who study before him,
as it is stated: “The beginning of wisdom is fear of the Lord, a good understanding have all who fulfill them” (Psalms 111:10).
It is not stated simply: All who fulfill, but rather: All who fulfill them, those who perform these actions as they ought to be performed, meaning those who do such deeds for their own sake, for the sake of the deeds themselves, not those who do them not for their own sake.
Rava continued: One who does them not for their own sake, it would have been preferable for him had he not been created.
Each person is expected to come to form their own relationship and find their balance between the two.
Rabbi Meir was wont to say the following idiom:
Study with all your heart and with all your soul to know My ways
and to be diligent at the doors of My Torah.
Keep My Torah in your heart,
and fear of Me should be before your eyes.
Guard your mouth from all transgression,
and purify and sanctify yourself from all fault and iniquity.
And if you do so, I, God, will be with you everywhere.
The story is told that after the passing of the great Rav Shlomo Zalman Auerbach, someone asked the family sitting shiva what the Rav used to focus on. They answered - keeping the Shulchan Aruch - the code of Jewish way of life!

Shlomo Zalman Auerbach (Hebrew: שלמה זלמן אויערבאך; July 20, 1910 – February 20, 1995) was a renowned Orthodox Jewish rabbi, posek, and rosh yeshiva of the Kol Torah yeshiva in Jerusalem. The Jerusalem neighborhood Ramat Shlomo is named after Rabbi Auerbach.[2]
BUT ...... that's not so easy. This task has been given to people!

Elijah ben Solomon Zalman,[1] (Hebrew: ר' אליהו בן שלמה זלמן Rabbi Eliyahu ben Shlomo Zalman) known as the Vilna Gaon[2] (Yiddish: דער װילנער גאון, Polish: Gaon z Wilna, Lithuanian: Vilniaus Gaonas) or Elijah of Vilna, or by his Hebrew acronym HaGra ("HaGaon Rabbenu Eliyahu": "The sage, our teacher, Elijah") or Elijah Ben Solomon Zalman (Sialiec, April 23, 1720 – Vilnius October 9, 1797), was a Talmudist, halakhist, kabbalist, and the foremost leader of misnagdic (non-hasidic) Jewry of the past few centuries.[3][4][5] He is commonly referred to in Hebrew as ha-Gaon he-Chasid mi-Vilna, "the pious genius from Vilnius".[6]
From the point of view of man's body and spirit he is lower than all other living beings, but from the point of view of his soul, he is greater than all of them. Even Sheleima 1:7

Feb 2019: Arnoldas Pikzirnis, Mr Skvernelis’s national security advisor, told AFP that the request to transfer the remains to Israel had been denied, because the “Vilna Gaon is an inseparable part of Lithuania’s Jewish community and Lithuania history.”
To be or not to be ........... a mentch!

or

In order to learn and fulfill the Torah, we have to confront the person who is going to do it --> Mussar is the Torah of the Self
Turn form evil and do good, seek amity and pursue it.

Mussar is the "turning" / refinement of oneself
The human issue is the issue of character
ונמצא כי יותר צריך ליזהר ממדות הרעות יותר מן קיום המצות עשה ולא תעשה כי בהיותו בעל מדות טובות בנקל יקיים כל המצות:
We, therefore, find that greater caution must be taken to safeguard oneself from bad character traits, than even from fulfilling the positive mitzvoth or desisting from committing the negative mitzvoth. This is because if a person possesses good character traits, he will not find it difficult to fulfill all the mitzvoth.

THE MUSSAR MOVEMENT

Yisrael ben Ze'ev Wolf Lipkin, also known as "Israel Salanter" or "Yisroel Salanter" (November 3, 1809, Zhagory – February 2, 1883, Königsberg), was the father of the Musar movement in Orthodox Judaism and a famed Rosh yeshiva and Talmudist. The epithet Salanter was added to his name since most of his schooling took place in Salant (now the Lithuanian town of Salantai), where he came under the influence of Rabbi Yosef Zundel of Salant. He was the father of mathematician Yom Tov Lipman Lipkin.[1]
And like all movements .........
In 1897, Eliezer Gordon of the Telshe yeshiva hired a new Musar supervisor, Rabbi Leib Chasman, who instituted a very strict Musar regime in the yeshiva. Many of the students opposed this approach, which caused dissent among the student body. At the same time, dissent against Musar also broke out at the Slobodka Yeshiva. A group of Lithuanian rabbis then published a declaration in the Hebrew newspaper Ha-Melitz in opposition to the study of Musar. According to the YIVO Encyclopedia,
they argued that while the study of moral texts was a venerable if distinctly limited element of Torah study, the sainted Salanter himself surely had had no intention of overturning traditional priorities and certainly not of creating a new sect that was itself contributing to that collapse of traditional Jewish life which it claimed to combat. This set in motion a wave of similar declarations, counterdeclarations, and polemics for and against Musar in the Hebrew press which reverberated throughout traditional circles. Eventually a sort of equilibrium emerged, with Musar remaining a feature of many yeshivas and its most heartfelt advocates and opponents finding for themselves distinct but congenial venues.[11]
In 1897, there was opposition among the students to the Musar method, and the yeshiva was divided into two. The followers of the mussar movement had to move to a separate building, while its opponents founded the Knesses Beis Yitzchak yeshivah, named after Rabbi Yitzchak Elchanan Spektor.[2] Rabbi Finkel, a great proponent of the mussar movement, led the Knesses Yisrael faction, and Rabbi Moshe Mordechai Epstein remained loyal to him, maintaining his post at Knesses Yisrael, despite being offered the position of rosh yeshiva in Knesses Beis Yitzchak.
