From the stage I can tell that she can't let go and she can't relax.
And he's never been good at sleeping alone. Maybe he's the type that can't sleep without enough sex and maybe he isn't really getting enough now, so the idea of leaving his marriage makes him feel like he can't breathe.
And he's realizing he's actually gay or wondering if he is and he has no idea how to come out of the closet and try to find a male lover, which makes that feeling of "I can't breathe" escalate to something that feels like the vacuum of space sucking the life out of him when he contemplates leaving.
So he mostly doesn't. It's become unthinkable because it seems like something unsurvivable.
From what I gather, internet dating sites have made it easier for the LGBTQ crowd to find romantic partners. Making that initial contact via internet helps keep it private and helps resolve a lot of the thorny issues surrounding trying to figure out if someone is just being friendly or actually flirting and if it would be okay to hit on them, given that you are both the same gender.
So this may be a much easier problem to solve than it seems like, especially if you have been with someone a long time and the last time you initiated a relationship was before online dating was much of a thing.
The reality is that trying to keep a white knuckle grip on yourself is probably the worst thing you can do. It tends to magnify problems.
The more you tighten your grip, the more star systems will slip through your fingers
Being alone ends up not being as bad as they feared because the real reason they felt so needy is the relationship was bleeding them emotionally, like a vampire. Once that stops, they stop feeling constantly like an anemic person in need of a transfusion.
If there are children or other complications, it may not be so straight forward. But if the biggest thing keeping you there is simply fear of being alone or fear of the unknown, it's often the case that simply leaving is the best fix for those things.
You can't know ahead of time how you will feel after it's over, but if the relationship itself or if being in the closet are making you dysfunctional, it's a fairly good bet that simply leaving will resolve a lot of the tensions that currently seem unbearable and are currently making you feel like you just can't cope.
Let me repeat that: You can't know ahead of time how you will feel after it's over.
So if you are catastrophizing about how terrible it will be, that may be your current high stress levels talking. And those stress levels may be primarily about being in the closet and/or being in a relationship that fundamentally doesn't work.
You might try reading Mom, I Need To Be A Girl. Once she transitioned, a lot of her school performance issues and the like resolved because the amount of energy it took to hide who she really was had been the primary cause of her performance problems.
The antidote to the emotional white knuckle grip is to let go. An open palm holds more water than a grasping fist.
The way you get love is not by being grasping. It's by being receptive to it.