Chafetz Chaim: chapter 1 paragraph 10
If someone violated the laws against speaking Rechilut, and he wants to repent (“do teshuva”), he has not even achieved partial repentance until he requests forgiveness from and appeases the person he wronged. In the case of Rechilut, this would be the subject of the Rechilut, the person spoken about, since the talk encouraged the listener’s animosity against the subject. He should also repent for violating the commandment “Lo telech rachil b’ameicha,” do not go about as a talebearer among your people, by repenting to G-d.
In Hilchot L”H 4:12 and the footnotes there, the Chafetz Chaim mentions the possibility that the sinner may not be allowed to ask forgiveness of the person he wronged. If the person who was spoken about does not know what was said about him, or doesn’t know anything was said about him, it would cause that person anguish to hear about it, which is a violation of “Lo telech rachil b’ameicha” (Lev 19), the commandment not to speak Lashon Hara or Rechilut. This is frightening, as it means that in many cases there may be no repentance for Rechilut.
I heard it suggested that it may be permissible to tell the person “I said something about you,” and ask forgiveness on that basis, but it would depend upon how the person would react to the information. A Rabbi knowledgeable in the laws of proper speech should be consulted.
In any case, the speaker should go back to all the parties he spoke to, and retract what he said. This may mean finding a reasonable explanation for what happened rather than the more slanderous interpretation offered originally, or explaining that the speaker didn’t know what he was talking about (it was a rumor, he didn’t witness it and there were other possibilities, etc.).
The Chafetz Chaim likens speaking L”H to ripping open a feather pillow in the wind – the feathers spread so quickly it is nearly impossible to recollect them all. And the longer one waits to retract what he said, the more the information spreads and the more damage is inflicted on the subject of the Rechilut or L”H.