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She's hurting you because she's hurt. But that doesn't mean she deserves your sympathy and understanding.
I generally agreed with the advice, except for this part. Maybe it's because I've been in her position -- but I can't help thinking about how agonizing this situation is for the ex. She loves someone that she can't have, and she must regularly witness his happiness with someone else. Since she shares the same circle of friends, she must either give up that circle or face a painful situation every time the group meets.
Her behavior is still inappropriate and wrong. You are completely justified in responding to her insults with "Fuck you," or whatever you want to say. But, I still wish you could feel some empathy for her situation. You are essentially in the privileged position. You get to be with the man you love, while she is alone and hurting.
It sounds like it would be best for all parties if you avoided the ex as much as possible. But when you do have those encounters, remember that nothing she says can change reality. You are secure and happy, while she is pathetically insulting the girlfriend of a man that she can't have. In other words, I'm suggesting that you should feel sorry for her. Don't worry about her comments. She might lead others to believe that you did not have a prestigious education.. the horror! Instead, remember the reality that surrounds those comments.
Yes, you should speak up for yourself. Talk to the ex. Attempt to escape her behavior, or tell her to stop it. But don't let her behavior upset you, to the point where it's a big problem in your life (advice column worthy). She is the one who should be writing a desperate letter to Cary. You should feel fortunate that you are with the man you love, and focus on that.