Just an update - WS and I sat down with our teen (nearly 16yo) today and told her we're separating.
I'd written a minimal note to say that WS and I have been dealing with challenges in our marriage for a few years, that we'd tried counseling, but it wasn't working out, so we're going to stop living as a married couple. We assured her that nothing would change in her daily routine, that we are on good terms and would do our best to co-parent her.
She said she wasn't surprised at the news. In one of her therapy sessions last spring, they talked about how my being in a rental with her during the week was nearly a separation from WS, so has had some time to consider it. She didn't offer up much detail, said she was feeling fine about it, and after a few minutes, went back to her room to keep playing a video game with a friend.
WS and I were very relieved that it went so quickly and smoothly. I'm anticipating that there could be some longer term fallout as our kid really processes it, but I don't think it will be severe, thank goodness! WS was a bit stunned by her response (as usual, his anxiety about it all was through the roof, though all of the disclosures to family and friends have gone better than he expected). I wasn't sure how our kid would react, but I had anticipated that "Obviously, mom and dad," might be a possibility.
So that's it. Now everyone important knows. I can tell local friends and extended family about what's really going on. And now we start the work of actually splitting up all our property, finding a mediator, and doing the paperwork.
To paraphrase Eliot, this is how my marriage ends, not with a bang but a whimper.