רבי דוסתאי בר ינאי משום רבי מאיר אומר כל השוכח דבר אחד ממשנתו מעלה עליו הכתוב כאלו מתחיב בנפשו שנאמר רק השמר לך ושמור נפשך מאד פן תשכח את הדברים אשר ראו עיניך (אבות ג:י)
Rebbi Dostai bar Yannai says in the name of Rebbi Meir, "One who forgets something from his Torah learning, the pasuk considers it as if he bears guilt for his soul, as it says, 'Beware and guard your soul exceedingly lest you forget the things your eyes have seen.'"
Rabbeinu Yonah explains that our Mishna is discussing someone who forgets his learning due to a lack of reviewing the material that he learned. Since it is human nature to forget, he should have reviewed the Torah again and again to the point that he would never forget it.
Rav Wolbe writes (Alei Shur vol. II p. 163) that people understand that one must review the Gemara that they learned. Being that there are so many details and nuances that make up a sugya, it is imperative that one review the material to enable him to remember everything that he studied. Yet, when it comes to learning mussar people have difficulty understanding the need for review. Isn't the standard mussar idea straightforward and succinct? How many times does a person have to review, for example, "Jealousy, lust and honor remove a man from the world" (Avos 3:28) before he is able to remember it?
The need to review a mussar idea is not for the goal of remembering the words. Rather, the review is necessary to ingrain - an already known idea - into one's heart and mind. Just knowing that "jealousy, lust and honor remove a man from the world" won’t do very much for a person who needs to uproot these negative traits from his character.
Rav Yeruchom Levovitz zt"l writes that contemplating an idea can be compared to taking a quill and running it over a paper. The first time this is done it will create an ever so slight indentation. With each successive time, the indentation will get deeper and more discernable. In a similar vein, with each reflection upon the idea that one previously contemplated, the impression upon the person's mind and soul becomes deeper and more recognizable.
Indeed, it was this type of learning and reviewing mussar that Rebbi Yisrael Salanter had in mind when he promulgated mussar b'hispa'alus. This exercise is not a means of brainwashing. It is a means of taking something which he already knows to be true and making it part and parcel of the very fiber of his being in a way that will positively affect his behavior.
Accordingly, we can explain that our mishna is referring to one who fails to implement mussar ideals. In effect he "forgot" what he learned because he didn't review it again and again. He should have realized that human nature is to forget and overlook even important concepts - if they aren't hammered into their mind and heart time and time again.
When Rav Yaakov Galinsky arrived in the Novardak Yeshiva, he was sent by the Rosh Yeshiva to learn mussar in the local shul. He arrived at the dark beis medrash and heard a man reciting again and again the Gemara (Eiruvin 54a), "Grab and eat, grab and drink, since the world we live in is like a fleeting wedding" (i.e. grab as many mitzvos as possible before it's too late). He later found out that the man was Rav Yisrael Yaakov Kanievsky (the Steipler). This tzaddik certainly knew that man has limited time in this world; he was merely trying to inculcate this reality into his heart.
A practical suggestion to help implement this idea: Take a mussar idea and repeat it ten times a day. It could even be something in English like "A person never loses out from giving in." You will be surprised to see how much of a positive effect this exercise has upon you even after merely a few days.