Mya - Some similarities between this and my recent experiences. I totally understand why you'd feel driven to processing this boundary now. It's emotionally "come up" for you, even though it's not reality. Seems like you'd be bothered by it, if you didn't feel and think it through right now.
I guess one benefit of the lack of urgency (cos it's not happening right now) is you can take it slower. You don't have to figure it all out at once. Some people would have a tendency to put it off, until they "had to" processes it. You don't seem like one of those people. So, yes, remember you do have breathing space.
This is big stuff. I've been the partner on the other side of something like this (i.e. rory's position) and I wish we'd both spent more time understanding each other, especially me understanding the shape of the boundary he was feeling, as in, what exactly was triggering.
rory - (NB: Forgive my tone here. I'm writing quickly before I have to head to work. I don't mean to imply that you don't know this already; I can tell that you are sensitive about it!) This is a boundary that Mya does not want to have, on her partners. She clearly wants to put serious working into making her emotions conform to her ideals - but this isn't simple! You can't just make your emotions do things, through a sheer force of will. Even teasing out where your fears are coming from, tracing the roots of the emotion, doesn't necessarily help ease the physical panic of an idea.
And - obviously - trying to rush "being okay" with something can be self-defeating. Yup, she's processing... and the hope is that things will ease eventually. But what if the boundary cannot shift?
For me, if I felt a partner has sincerely tried, and is trying (within reason), to work on a boundary that neither of us intellectually want, that means a lot. To me, it feels like the difference between a boundary which is an uncritical projection of fear, almost a lazy thing, and a boundary (however temporary or permanent!) that is more like a mental health issue.
I know it's not possible to assure a partner that you will be with them, no matter what. But, as well as supporting Mya emotionally while she's processing this stuff, you could also think about the ways you'd be willing to go slower, or even of some interim or potentially permanent boundaries you'd be able to accept, in consideration of the way Mya says she is triggered by some of this. Be careful here... If you do agree on boundaries, these are likely to be less flexible than other boundaries around things less emotionally volatile.