That lends to the FOO theory.
By and large, that's the answer. Although sometimes there is independent an psychological issue at play a well that wasn't predicated upon one's FOO and early childhood.
My advice is like so many others, if they want to leave, let them go. Focus on you and here's the kicker that I don't think gets enough merit on this forum, also focus on your own FOO issues.
Now granted, yes, many of these wayward spouses (both male and female) were people who showed up every day with a smile and had people believing they were mary poppins and mother theresa all rolled into one who suddenly exploded and if you're here now reading this, I'm sure you've awoken to the idea that wasn't the reality at all.
So while yes, it's possible that these people totally fooled us, I think it's quite more likely that our own FOO issues is what allowed us to be fooled in the first place by blinding us to red flags or by not giving us the guidance and experience to recognize what a red flag even is.
So yes, GAL, focus on you but also really focus on your own FOO and how they contributed to things. As others have said, you've got a lot of time to think and you should learn to use it constructively to envision what a healthy, functioning relationship looks like.
And if you ask me, it's two independent, self aware individuals who are responsible for meeting their own needs, emotionally, financially and otherwise, regulating their own behavior and also taking responsibility for their own actions, reactions and also inactions. Learn about healthy boundaries and attachment theory as well.
And yes, you need all that attraction, fun and other things too and I'll tell you what, kindness and emotional availability are also very important.
It takes time and give yourself plenty of it and self love and kindness too, because it's a lot to unpack and I'll tell you what, do it sooner than later.