Sigh. I'm very sorry you are going through this.
You state several times you are willing to try. If you aren't trying to foot drag but are lacking in support/education/nurture to help you transition well from a Closed thing to an Open thing on top of new motherhood? And as the poly person he's like "Just lump it on your own. And don't be sad at me because I cannot deal with negativity" -- that's just not right.
The kid is not even a year old right? Could you as the mother have post partum depression? You def. are isolated. And you are struggling with Opening on top of new motherhood.
Is this REALLY the best time to be Opening anyway? How can he tend to other partners when he doesn't tend the one he already has well?
Something is just not right there.
His freedom to do whatever, whenever is more important to him than any one person, I think, and if he feels like I am asking him to do things that restrict that freedom, I don't think I have a place here.
Perhaps you are right.
Best to call your mom and get yourself out of there and move toward healing space for you
. You are not comfortable with being Open, and you certainly are not going to get better at it with a partner who treats you with such little concern.
Hang in there.