"I am exhausted from trying to hold on and keep everything together. It feels like I'm the glue, and I'm not even attached to anything. How do you know when it's time to walk away and hope for a better life by yourself?"
I'm sorry. I'm so, so sorry.
And I know that exact exhaustion.
I would suggest that everyone who is feeling like ending a marriage read the excellent Uncoupling by Diane Vaughan. It is NOT a book that will make you feel bad in any way about "what you're doing" or your kids, so I consider it a totally safe book. (Also safe for same-sex couples.)
What it does is it goes through the entire emotional process that happens when a couple separates, starting from the very first inkling one person has that things aren't right. It talks about all the places the mismatches happen, and who tends to feel what when. It also talks about points at which the relationship could be saved if the partners are both willing and able to identify that and change behavior.
It's really just a timeline of how things unravel, from both sides, of the person who leaves and the person who's left. I found it invaluable.
Now, from personal experience, I had all sorts of different points of knowing. Everything from realizing that when I imagined myself at the age of 50, he wasn't there in my vision, to sobbing through church every week because my life was so wrong, to thinking Nora Ephron's Heartburn was the best book I'd ever read. (It's a good book, and the recipes are amazing, but I don't think people in healthy marriages identify with it as completely as I did when I read it.)
The bottom line for me, though, was that at a certain point leaving was the only thing that made sense. I knew it would be hard (I thought of it at the time as chewing off my own foot to get out of a trap), but it was the only path I could imagine. Staying seemed impossible, and like sickness.
Anyone who left, how did you know?
Anyone who fixed it, how far did you go before you pulled it back together?