I have bipolar disorder. I treat my girlfriend badly sometimes, especially when im manic. But just being bipolar doesnt give her a freebie to treat you like ****. I do it too, but it isnt right. And my mother, whos also bipolar, does it to me and our family. So its normal. I get that you can only be understanding to a point.
Try talking to her about it in a calm moment. Or you can get with her (or get your own) therapist to swap ideas and have someone else say something to her about this. Is she on any meds?