The following is from a Torah Tavlin parsha sheet (M'tzora 5774):
A famous story is told about the holy Chofetz Chaim
ZT”L, who sat next to a Jew on a train traveling to Radin.
The traveler was unaware of the identity of the man sitting
next to him and he mentioned he was going to see the great
Tzaddik of Radin. R’ Yisroel Meir protested and said that the
rabbi of whom he speaks is not so special. Enraged by the
disrespect shown by his fellow traveler, whose identity he
did not know, the traveler smacked the Chofetz Chaim.
Later, the Chofetz Chaim set the man’s mind at ease. “I
deserved to be beaten,” he said, “because, as I have learned,
one may not speak lashon hara - even about oneself!”
Notwithstanding this story, one may wonder as to why this
should be the case. Seemingly, one’s feelings are his own.
Just as one is permitted - although not encouraged - to
embarrass oneself if he so desires, presumably he should
reserve the right to speak of his own shortcomings as well.
Indeed, R’ Yosef Sholom Elyashiv ZT”L is cited as
remarking that no prohibition exists whatsoever against
verbal self-denigration. One may, in fact, speak lashon hara
about oneself. R’ Elyashiv commented that to the contrary,
the above story proves this position, for we see that even the
Chofetz Chayim permitted himself to speak about himself.
He would not have done so if it was strictly forbidden by
halacha. Only in an attempt to alleviate his assailant’s
humiliation did he make the comment that he was wrong for
talking about himself. In truth, however, it is permissible.
Nevertheless, R’ Elyashiv insisted that this does not grant
other people permission to speak about him. This is
compared to a situation where one lets another steal his
possessions. Undoubtedly, though, the thief who comes
along and seizes his property has violated the prohibition
against stealing. Similarly, the fact that a person speaks freely
about his own inadequacies has no effect on the prohibition
forbidding others from engaging in such talk about him.