^Yes, thank you for the clarification.
I think for me, I have felt along these lines about boundaries for some time. However, I am reflecting on it also because Mya is starting to date. It is a somewhat new situation for me that my partner is open to finding other partners and actively doing something towards that.
I had a few moments of anxiety, when she first started, which was coming from fear of change. I wouldn't have been surprised to have some more (and probably will), but I haven't experienced any since then. I feel it is largely due to this processing. There is really nothing to fear. I trust that I will remain important in her life and she will make choices based on that. But, even more importantly, if she doesn't (because nothing is ever 100% sure), I can take it as it comes. If it happens, it will hurt, but I expect that I will be able to accept it and move on in one way or another.
It isn't even the case that I think I will be able to survive anything. It is not an infinite belief in my own abilities. It is just that for me the comforting thing is that whatever happens, I will have choices
. Even if the choices are something I am not too happy about, they will still be mine. That, I think, ties in with the thing about relationship rules/boundaries. Rather than ensuring my safety (because, in the end, nothing is certain), they limit, they take away some potential future agency. They take away my power to evaluate the situation and react based on that, rather committing me in advance to being hurt.
If a partner breaks my trust, I want the freedom to assess and choose the extent to which it affects me and our relationship. I refuse it to be predetermined by rules, even ones I've set myself.
[This, btw, is one of the most harmful things about the cultural concept of cheating - it determines sex with others as something that is automatically hurtful, harmful, as "the worst thing that can be done to another". It strips the person who's been cheated on from the agency of deciding, themselves, the meaning it has to them.]