I'm sorry you are both in such a difficult situation.
I have a little bit of experience with this, as TGIB was in an abusive marriage that was ending when he and I started becoming more serious (we had been friends forever, even before he met the woman he married). But our situations are very different, since I was already married and any relationship between TGIB and myself was automatically not a monogamous one. So I may not be able to completely speak to your situation, but I'll try.
Coming out of an abusive relationship, she is very fragile. It's possible (even likely) that she doesn't even realize how fragile she is and how much she is lashing out. Just as you are trying to prevent yourself from being hurt, she's trying to prevent herself from being hurt AGAIN. Unfortunately it's going to take time, and probably a lot of it, for her to get through this, especially if counseling is unlikely to be an option for a while because of her ex's behavior (that really is despicable on his part). Even when she thinks she's reached more solid ground, something unexpected could pop up.
I'd recommend two things to you. Stay calm and gentle as best you can and try not to take her anger personally. YOU know you aren't a fraud. She will realize that eventually. Try to maintain a consistent position of, "I love you, but we aren't and can't be exclusive right now. When you're ready to discuss this further, I'm here." (or similar) Perhaps try to let go of labeling whether you're poly or single and just leave it at "non-exclusive" The other thing I'd recommend to you is to try to find an online or in person support group for people with loved ones coming out of abusive relationships. It can be a VERY difficult position to be in and if you want to be there for her through this you could probably use some help and support yourself. Since you can't change her behavior, focus on your behavior and what you can do to be the healthiest YOU.
I wish you both luck getting through this.