You clearly are not happy in the relationship, you've tried to end it but you've allowed him to change your mind. If you don't want to continue on this path, you'll just have to be more firm with him.
It's one thing to be poly. It's another thing to be a pick up artist. It's another thing entirely to use that as an excuse to neglect the feelings and needs of people you're dating.
It takes two people to be in a relationship. It takes one person to end it. He does not have the power to "refuse" to end it, unless you give it to him.
Send him an email, along the lines of "Yeah, I could probably get over these feelings. But really, you're just not worth the trouble. You don't show consideration of my needs, you don't empathize with my feelings, and I don't have time to waste on someone like that. Besides, why should I be the one to change? There's nothing wrong with me, we're just not compatible. Have fun with your pick-ups, but I'm out." Then the rest is on you: don't answer his phone calls, don't reply to his messages, and for the love of Spaghetti, don't contact him yourself.
I happen to agree that humans are not monogamous "by nature," i.e. that we're not genetically programmed to be monogamous. That being said, socialization is an incredibly powerful process, and it has absolutely made many people unwilling to engage in non-monogamous relationships. There's absolutely nothing wrong with being one of those people, with only seeking out partners who feel the same way you do, and with not doing the hard work required to get over the jealousy and insecurity that's triggered by sharing partners.
You deserve to be happy. You deserve to date people who approach relationships the same way you do. You don't have a responsibility to change your attitude, personality, and preferred relationship style just so he can play his games without consideration of the effect on people around him. That's crap.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker