I am going through something similar.
This girl I have been seeing for nearly 2 years, she knows that she won't make a boyfriend out of me. I continue to see other girls in between and she will give me shit every time she finds a hair clip, piece of glitter, blonde hair in my bed.
She has dumped me about 7 times now and each time I stick to my guns and I don't break.
2/3 days I will always get the ' I miss you ' text.
I fuck her sooo good that can't commit to dumping me because she ain't gonna find a guy like me in a long time.
I feel like a cunt sometimes, but at the end of the day I was honest from day 1 about being Poly and I do enjoy her company and I devote all my attention to her when I with her.
When a girl gives you shit like this, i think you show integrity and stick to your guns no matter what, you have to hold the line like this:
https://www.youtube.com/watch?v=N1NupxasQWs
When she gives in, the sex is angry and rough and they love to be degraded for being a pain in the ass. They love it.
I could get rid but I don'y see any point in making both sides unhappy, she is free to hook up with guys but she doesn't want to.