Well, after 17 years together, my wife and I have split. Reading many of the recent posts above made me want to write about it. I knew this was coming for months, I brought it up last year but we kept trying. In March I told her I wasn't happy and wanted to separate.
COVID has been rough. Our son (she is not his biological mother but has spent more time with him that his mother) developed a meth and sex addiction (I am told these very often go together), and I had to remove him from Santa Cruz and bring him to LA because he set his apartment on fire in Santa Cruz. He was introduced to meth and sex combined by a Santa Cruz police officer who is a prostitute on the side. He's in a rehab in LA and doing well now, been sober four months and has a great support in the facility. But for the first year he was in LA he was with us, then in rehab, then he'd take off and was on the street in LA off and on in emergency homeless shelters, he checked himself into the hospital once or twice just to get off the street. Got into some hairy situations, and finally had enough.
About four years ago I had a brief affair. It was mid-life ego-stroking bullshit, and I feel horrible about it still. It was the result of a confluence of events and life changes all happening at once, combined with my insecurity/needing adoration. She never got over it. Maybe I didn't either (guilt/shame). I still love her, always will. Probably will not look for a relationship of that sort again, definitely will not marry again.
She moved to SF last month. I didn't want her to leave LA, but she has a lot of friends up north and none here. We still talk daily, she really lets me have it about past behaviors on a daily basis, mostly by text. She has a lot of issues surrounding her upbringing that she never dealt with I guess, she is doing that now. Some people just take longer to let that sh!t go. We were both raised by parents high on the narcissism scale, probably. I'm sure I am up there, but hopefully not as much as mine. Saul Bellow has a great quote about parents/upbringing that has always stuck with me from his short novel, A Theft, "...after the age of forty a moratorium has to be declared--earlier if possible. You can't afford to be a damaged child forever."
So yeah, many reasons why people are single in mid-life. I don't judge.