I've just read through this story and it makes me cringe. Red, please stop blaming yourself! I'm guessing your pain has less to do with his polyamory and more to do with his neglect. It's not that he has feelings for his girlfriend, but that he is not demonstrating any love or commitment to you. What kind of new father spends nights away from his baby in the first 2 weeks?! Or when you were pregnant beyond your due date?
Seems to me like you might be perfectly willing to accommodate his polyamorous tendencies if he would first fill your own needs. I don't see any reason why he would expect you to be ok with his going off to be with his girlfriend is he hasn't massaged your feet, changed the baby, and given you a chance to nap first. And that postpartum ban on sex? That's only for penetration. You are plenty entitled to orgasms; in fact they release oxytocin, like breastfeeding, which is good for uterine health. Surely he knows some creative ways to get you there.
He doesn't come across as respectful or considerate in what I've read here, so perhaps you'd be better off without him, but I would still hope that you wait until you've adjusted to your new baby before making any permanent decisions about your marriage. For her sake, if nothing else. If he thinks he has it in himself to be polyamorous, this is his chance to prove he really does have enough love for more than one. (I'm married and poly but I never let time with another man take away from my husband; my first priority is to give him all the love he needs and then some.)