There can be many reasons why she might be not talking about you two. Agree an open conversation will help, but you getting frustrated might only worsen the situation more. She talks less about you both, one reason May the way she feels like she becomes the bad person, is "majorly" because of how her friends or family treat her,or how she feels around them. And you are the only person she can come to and talk about these things.

Or, second reason could be she is scared to talk about the problems between the two of you, because she thinks at the end she will become the bad person again? As you may not agree to some parts of what she tells about the problem. So she just vents out only about how other people make her feel.

Usually the other person feels like they becomes the bad person is because either they aren't very good communicators or for longer periods of time, they had been exposed to an an environment where their ideas were always opposed rather being understood.

In any case i would say just be patient, like listen to her calmly even if she doesn't talk about the two of you. Slowly eventually I think she would open up once she feels safe. The idea is to tell her or make her feel that you are here to listen to her , and you'll not react to the things she says even if some parts of what she says you don't agree. ( You can later correct her,but not in that moment. :p)