This is again somewhat different, but I truly don't think that fear of losing US is what does it, it is something more internal. Now for some perhaps it really is fear of losing us, but for many I think it more fear of facing themselves.
Years ago a well-meaning person told my H that other men were interested in me; my H then wrote to say that at first that felt weird, but that he would now fully embrace this and "we" could go forward, being friends, etc.
I set him straight and said that there wasn't anyone else, that my choice would be to repair our marriage and family, but that was actually besides the point.
As I learned later, as more and more info on his crisis came out, it was more that he thought that if I had someone else it would justify everything he had done, and he wouldn't have to account for any of it, he figured he wouldn't have to pay, that it was all OK. A minimising of the impact of the huge trauma his behaviour had caused. That he had "got away" with it all.
If we really are done, then it doesn't matter if we spook them or not, we're done.
And I don't think there is any particular timeline on how long it takes for them to feel anything in particular; what I described above was done after something like 2 years; my own experience shows that it is still all inside them, even if we do, like I have, get to see some of the process at times. And I have seen all sorts of things in the past decade.
That, in my opinion, doesn't change the fact that it is still up to us to become the best person we can be, and that means doing what is necessary to keep our finances in order, look at our own behaviour towards everyone in our lives, and keep learning.
Perhaps by doing so we are a type of lighthouse, the port in the storm, but that also means having firm boundaries and learning how to communicate those politely. If I have learned anything is that appeasement never works, and while nothing may "make" our spouses find their way to us, if they do we need to be strong and worthy of respect.
And the lovely thing about that is that it truly does make us "better" in all areas of life, be it with our children, our jobs, our friends, and so on.
My own only answer to my H about why I don't have anyone else has been that I get to choose how I live my life, and that having someone else isn't the only definition of happiness.
Now my H may choose to view that as pathetic at the moment (I actually don't know anything about what is in his head any more), but that could also change, and I don't know what could trigger that change, but it is very possible that it could be something very little, something unrelated to anything else, and if that happens we will see what kind of person he is then.