He’s saying now that he has never been happy in our marriage of 20 years.
Ouch. I'm so sorry you have to hear this, and at the same time taking care of him after an affair. He's probably expressing pain and self-loathing but still. It's a lot to take in.
I’ve been reluctant to pursue a protective order, because I fear it might escalate him.
There's a great chapter in Gift of Fear by Gavin de Becker about POs and when they're useful, not useful.
Does he have any family in his life?
When I left my ex, it came after a year of planning. A lawyer had outlined what could happen so I took that info and got ducks in a row in case we ended up in a worse-case scenario. I didn't feel strong enough to just up and leave so I did a bit extra knowing it was necessary if I wanted to actually get out.
Things didn't go exactly according to plan when I left but there were so many pieces in place that it wasn't a total catastrophe.
In my experience, the heat of the emotions during an active abandonment don't usually end up in cooperation.
Have you consulted with an attorney?
Are you seeing your own therapist?
I'm grateful I had a team of people in place so when my ex went off the deep-end it wasn't just me and my internal dialog piecing things together. As someone with codependent traits, I had a tendency to over subscribe cause and effect to my behaviors, as though I played a more important role than I actually did. Having others involved breaks up that narrative and keeps things real at a time when you need that reality in order to move forward.