These days, you can take back all kinds of things, such as clothing, electronic equipment, or even a car...but you still cannot take back the spoken word. For better or worse, you are having a "learning experience." While such an experience is often a code-word for a failure, all that it means is that you failed in a particular situation, but you are not a total failure unless you repeat the behavior that brought you here in the first place.
In the meantime, there are some actions that you can take. On the preventive side, if you are ever in doubt as to whether you should say or do anything, picture how it would look on the front page of this newspaper the next day. Whether dealing with a business issue or not, Murphy's Law still applies: if something can go wrong, it will. And, don't forget Murphy's Corollary: not only will things go wrong, they will do so at the worst possible time.
As for what to do now, instead of looking at this as a bad dream, it makes far more sense to refocus things and recognize that this is a real opportunity to grow. First, take a look at yourself and try to honestly see what caused you to make the less than flattering remark about your fellow employee in the first place. By looking at what you see as a weakness in others, you stand a real chance of identifying a weakness in yourself.
Secondly, until you clear the air with this person, you will continue to feel uncomfortable. The best way to find relief is to seriously consider approaching her and apologizing. After all, if you're satisfied that you made a mistake, the best thing to do is to correct it.
While you and this person will not necessarily bond into lifelong friends as a result, at least the heavy air of tension for both of you will be dissipated and you can put your attention where it belongs: on your work.
And, at some point along the way, you may want to talk things over with your former friend.
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