Well............what you describe is pretty standard. It's a new skill we have to learn and takes some time. We learn from childhood that speaking openly and honestly often carries a penalty. We have to 'un-learn' that, and in order to 'un-learn' it, it has to be in an environment that can minimize any potential penalty. Otherwise our brain won't allow us to proceed.
But I think it's not even that simple............
There's personality involved. Or at least 2 of them
Some people by nature feel better living openly. It minimizes any hidden sense (potential) of guilt, uncertainty etc. If you lay it all out there and it gets 'approved', you feel validated, where otherwise you might feel unsure of your choices or feelings. Maybe some choose to call this a confidence thing ?
Some people have a need of more privacy just to feel they are owner/operators of their own life. Having a pressure to share too much feels like a control mechanism. For those type of people, openness pretty much has to come at what pace they are comfortable with. For those type people, it seems the best solution is for them to TRULY understand and embrace the importance of it. And I think you have to acknowledge that this is much more critical if choosing to live a poly lovestyle. There's more potential for complication, more people involved, and being left out of an important 'loop' can be fatal. Sometimes literally.
So have some good discussions about this. Discover each others 'styles' and commit to working on it as much as possible within the boundaries of your own personalities. Stretch a little at a time.
It's super important - and worth the effort. Once you get there it feels GREAT !