awwwww, poor lil pookie, I'm so sorry you're hurting. I'm completely impressed at the depth of your self-knowledge and understanding. You sound committed to your beliefs.
You are very likely still reeling from hormones, which are enough to make mono women feel like that after a pregnancy. (so I'm given to understand, can't speak from experience here)
Are you able to share your frustrations with her as well as your husband? When I read things like you're feeling like it's a lie to spend time with them, I'm wondering if you're holding back somewhere.
The phrase 'mandatory monogamy' struck me also. I'm wondering if you're wishing that you could have non-mandatory monogamy? There are one or two threads around here recently about poly folks who are missing monogamy.
Poly is a lot of work. I suspect it feels like even more work when you're all under one roof, and exponentially more energy when you've got a new infant in the mix. It's not beyond the pale to imagine that you're simply exhausted.
Wishing you peace...
Love is that condition in which the happiness of another person is essential to your own...
Robert A. Heinlein
Me: female, bi, (formerly hinge of a vee)
with FirstBoyFriend (FBF)(moderately long-distance)
and no longer with CurrentBoyFriend (CBF)(who now lives in a house far away-with stairs I can't climb)