I totally get not being able to have that conversation. It is very hard. But that shouldn't stop you from doing what you have to do to be safe and heal.
Write a letter. STBX and I never actually had the D conversation. I wrote her an email stating what I required to feel safe (NC, openness, etc), and I wrote right there if she couldn't provide this it wouldn't be a trial separation, like she wanted, but a Divorce. I also gave her a hardcopy. She complained that "it wasn't romantic enough" and said "let's just be friends".
I moved out a day or two later, while she wasn't home, without telling her. I'm sure she was angry. She probably still is. She filed first, but I had told her a specific date when I was going to file, so I guess it doesn't matter who filed.
However you do it, take care of your own self. Don't worry about him or what he says or does. Separate mentally - he already did.