Just throwing my two cents in here.. I think that at the root of it, the MLC'er got that way through being at some level, avoidant attached and for that reason alone, they are very, very unlikely to take an responsibility in repairing the relationship.
We all more or less agree that these crisis happen through FOO issues ad trauma and our spouses have spent the better parts of their lives avoiding dealing with their pain and issues and we have no reason or expectation to think that it will change in the future. The fact that many of these MLCers end right back up into situations they claim they wanted out of with us is proof enough I think or even the constant replay. They're so used to keeping on a mask, keeping everyone in the dark about their inner issues and when the jig is up, one way or the other, they just move on and continue the charade.
That's literally what happened to me. When my ex and I first got together, I woke up in bed alone to find her sitting in the dark holding a knife to her arm crying. Years later, I realized now I caused her crisis by quiet quitting my marriage. So she began cheating, got pregnant and got herself situated with OM and then she tried to kill herself again. I went an visited her in the crisis center. She was a crying wreck, sorry for everything that had happened, telling me everything was a mistake and that she didn't belong there and then when visiting time was up, she literally shrugged it off, became "normal" and walked back into the center. The transformation was mind-blowing and truly revealing of this kind of "mask" that they can wear. Looking back now, it was obvious, how many times she moved from family member to family member, job to job, boyfriend to boyfriend and so on, as she was exposed for who she was.
That was my situation, but anyway, my advice is is they want to go, let them. You focus on you. Live. Live the life you want to, that you should have, that you've always yearned for.. whatever it is, do it, be it. Don't let regret, fear or shame hold you back or keep you holding on. None of that stopped them from leaving. It shouldn't stop you from living either.