Have you talked to him about this? What sort of reaction did you get?
He can't help you or do more for you if he doesn't know you need anything. Yes, you are making an effort to understand and accept his needs. And that's fantastic! At some point, he told you 'I need the freedom to love another in addition to you,' or something similar. He expressed a need, and you are helping him fulfill it. You seem very frustrated that he is not helping to fulfill your needs. I can't help but wonder if you have told him what those needs are.
Because if you're having calm, non-accusatory, open conversation about what you need from your relationship, and he is simply ignoring it, that's probably a larger issue than just dealing with polyamory.
You haven't said whether you've met the second woman or not. Have you? I find people are a lot more intimidating as the mysterious 'other.' Meeting in person and realizing they have quirks and flaws just like the rest of us usually goes a long way toward calming my nerves.
(For reference, I am a polyamorous woman. I helped my husband come to terms with this, and he has realized that he is also polyamorous. We are new enough that there is still a lot of learning about ourselves and our issues before this is 'smooth.' I have started seeing a man who is very likely wired monogamous, as you seem to be. It's tough for him to understand that he is not replaceable. We talk a lot about how this is going to work in the long run. All three of us.)
I have a lot of love to give.