If you trashed someone behind their back, apologize to the people you said it to as well.

Talking about somebody behind their back does two separate kinds of damage. There is what it did to them, which is obvious, and there is what it did to everybody who heard it, since they now carry an unfair picture of a person they might otherwise have liked. Apologising only to the target leaves that picture exactly where it was.

Go back to whoever heard you say it and correct the record out loud: that was unfair, and here is what is actually true. Do it without asking them to keep it quiet. Then apologise to the person, with no explanation of why you said it.