I agree that it is still cheating. She is using "being poly" as an excuse for bad behavior. Being poly means honesty and trust above all, period. She isn't fulfilling either of those criteria.
I think you need to start from scratch, maybe wipe the slate clean, redefine your boundaries, and if she continues to disrespect you by not following those boundaries, then it is time to decide what to do long term. You can't continue to be treated like crap.