Hilchos Rechilus, chapter 5, section 7

If he already accepted rechilus, the way to fix it is to work hard on himself and put the story out of his heart.

Even if it is hard for him to think the whole story was made up, he should think: perhaps the speaker added or left out a small detail, and that detail turned a good story into a bad one.

He should decide that from now on he will not accept lashon hara or rechilus anymore, and he should confess. This fixes the sin — unless he has already told the story to others.