You talk a lot about wanting to be seen as more an equal to your metamour, having a role in parenting their existing and future children, and feeling betrayed that their family planning (having another child) was done without including you before moving forward.
And then you say you try to maintain a friendly rapport with his spouse.
Do you contribute time and money to the care and maintenance of their kids and home? If a family emergency comes up that causes plans to change, are you upset and feeling a loss? Do you offer help in shuttling the kids to activities? As asked above, do you offer to watch the kids so they can go out and reconnect romantically? Would YOU be happy if that was something your partner would be happy to do or would you find it threatening? If you and he wanted to move on to having a kid now, how would you feel consulting her and her having an equal say in that or how to raise that child?