Hi, Beccy,
Have you ever gone back to review your posts here, or anything else that gives you a record of what you have been feeling and thinking in the past few months?
I know, I always want to keep a journal myself but I never have done so consistently.
But my idea is that if you look back as best as you can--to documents that you actually wrote then, because of course it's hard to recreate ourselves accurately in retrospect--I suspect that you will see a picture emerge. What I see in particular is someone with a more and more solid sense of self, able to understand where you leave off and where other people begin. Yep, boundaries, and you're doing very well with recalibrating them, I think.
I also see someone who is doing better with being patient with herself and with the process of living. Sure, I sympathize-- when I'm miserable I want the discomfort to stop as soon as possible (like yesterday). But it seems to me that you're developing a sense of baseline stability: today is a bad day, maybe, but tomorrow might not be. And in any case, you can cope with what comes.
Yes, it's rotten, and yes, I feel for you in your heartache. But there is much about your life that is still OK. And you and your children will be OK, no matter what develops over the long term for you and your BF.
Ten years from now--heck, even two--you'll have an entirely different perspective. That might not make it easier to accept the uncertainty now. But remember to give yourself credit for having already accomplished many challenging things in your life. Here's another challenge; you can meet it.

I'm just a poor wayfaring stranger, a million miles away from home.