It's great when metamours can be good friends but that doesn't always work. Is there a reason why you have to be in contact with her? If she drives you nuts often enough to be a problem, then stop contact or cut back dramatically on calls, texts etc. Remove yourself from the drama. Don't hang out with crazy making people.
That said, it reads to me that you don't really have a problem with her. You have a problem with your boyfriend's response to her, as those responses apparently come at your expense. So the problem is with the boyfriend. Is he breaking off dates to attend to her? Not fully present when with you because worried about her? Some other situation? He doesn't have to 'see it' the same way you do. He does need to understand that his behavior (his behavior - not hers) is disturbing you to the point of breakup. He doesn't need to agree with you that she's an attention seeking missile. He just needs to understand that his behavior is affecting you negatively. You need to communicate exactly what he is doing to make you feel lesser or ignored or whatever is coming up emotionally. From there you two can negotiate a shared solution that makes you feel more secure and loved and doesn't put pressure on him to accept that he is involved with a narcissist. In other words don't focus on her. There is nothing you can change there and don't focus on their relationship - even if it is batshit dysfunctional. Again there is nothing there you can change. Focus on him and your relationship with him.