Sally, is this triggering something for you, or re-traumatizing you in some way?
I don't ask that rudely, just that since you are feeling so overwhelmed by these 'mean girls', etc, it does make me wonder if you had some unfinished business of your own in this regard.
Sometimes, parents feel their kids are an extension of themselves and thus, feel hurt when their child is being treated in a shabby way. Other times, parents have their old baggage of their teen/tween years and their kids' experiences can trigger old hurts which weren't resolved at the time. (these are different situations, of course, and should be addressed differently.) They are still in there, sometimes, and our inner teen can be hurt by stuff that's not directed at us at all.
Maybe do some journaling to figure this out for yourself. Go talk to someone. I am not saying that it's not right to be hurt or angry when we feel our kids are mistreated, however, you do want to make sure you are listening/advising from a healthy, more objective place so that your daughter can receive good guidance which is in proportion to the situation.
My son isn't a teen, but we've already had some drama. I just try to keep his complaints in check, try to listen, offer a couple things he might try, and also encourage him to 'just see how it goes', because kids this age can be very fickle-- nice one day, upset with each other the next, and then fine again for quite a while. I also try to help him see *his* part in the problem. Part of what we parents tend to respond to is our child's level of distress as opposed to the actual situation. I hope you can find a way to gain some objectivity and feel comfortable shrugging off those insults when your daughter is able to instead of having those hurts linger.