Please know what happened to your H is not about what you did or didn't do or even really about you at all.
MLCers cannot face or deal with the issues that truly fuel the crisis. They are the things that happened to your H long before you ever met. Things that he didn't have the tools or knowledge at the time to deal with them. So, he has suppressed them so deep he doesn't even realize that they are at the core of his crisis.
If your H is like most MLCers, they avoid dealing with issues that bother them. They believe if they stuff them down they'll just go away. It has worked for them before, right? In midlife transition when humans typically take stock of their lives, the MLCer sees what he/she thinks they've missed out on and rushes out to try and capture that before it's too late. They try and outrun all that they believe has prevented them from being happy. That includes spouse, kids, and responsibility. When the transition goes off the track and derails, that's when it becomes the crisis. They are willing to throw everything away because they have no idea how to deal with those issues that they've put a tight lid on all those years before and it will be a long time, if ever, that they recognize that this really was behind what drove them.
They are being driven by emotions and their belief that what's bubbling up has to be the fault of their trappings in life. Because they have become so adept at not dealing with issues they run to find band aids that temporarily soothe them and calm their fears.
Eventually, most MLCer's do realize that it was not their spouse or kids that caused their unhappiness. The depression follows them no matter which 'new' life they've chosen. Until and unless he deals with his demons, they will haunt him and he will continue to look for someone or something he can blame.
Detachment is your friend. Own your part but DO NOT let yourself take the blame for things you had no control over. Change those things that YOU don't like when you look into the mirror. Make those changes for you. What he's spewing about now and blaming you for has little to do with what his real issues are.
Let those things he spews roll like water off a duck's back. You will get through this and you're not alone, we're walking the same path right beside you.
