Okay, I got it now, Nightwalker is male and his partner is female. Not that it matters, but it makes sorting out the pronouns easier.
I'm going to go out on a limb here and ask, Nightwalker, did you feel emotionally blackmailed or pressured into agreeing to the poly arrangement?
Polyamory is not a tool for fixing relationship problems. As someone else here said, that works about as well as people having kids in order to fix their relationships.
Many of us are showing concern for the new partner. This is someone who got sucked into the drama between you and your partner. That sucks, and is unfair.
You must always treat others as an end unto themselves, not as a means to an end.
But, yes, your child comes first. Your partner has a duty, and she has to figure out how to fulfill it. The new partner might well end up getting the short end, and I feel for this person. You should too, and try to have compassion as you work to figure things out.
Sorry for the double post.