Welcome! Sorry to hear you are struggling tho.
Are they feeling guilty for the start? Where you didn't really get a voice in it and now they are overcompensating?
Or is it dealing with the disappointment that it is NOT going to be a triad?
You like her and seem ok with them. You just don't want to be with anyone romantically but your husband. That's fine. You want what you want and there is NOTHING wrong with that.
Could tell them BOTH when you talk as a trio something like...
"Look, I am fine with you guys being together, but this is a "V" configuration. Hubby is the shared romantic sweetie. My metamour and I are just friends. I like her/you. Please stop trying to make it be more than what it is because that makes me feel not heard and my own wants for my romantic life not respected. If this is disappointment that it is not a triad -- let's deal with that together. But that doesn't mean PUSHING me to something I do not want. I want to be a good "V" partner. Please be good "V" partners BACK. "