I think all of this is something in need to keep in my pocket as far as letting my walls down. I've been editing the letter a little every day. I'd like to get to where I'm not flat out saying you're depressed and in a crisis, get help, because moving out and OW will never fill all the holes the misery you're in has created. I won't wait forever for you to figure this out, and you're going to end up dying alone if you don't. I'm gonna hazard a guess he wouldn't be that receptive to that. I need to make it more about me and how I feel and what I think, without anger and saying things I know he has to figure out on his own.
Here's a suggestion. Put the letter aside and start a new one-- this one focused 100% on YOU. Where you are, how you feel, your fears for yourself and your girls, your wishes and dreams for yourself and your girls (that might include him, or might not). I wouldn't censor yourself from a DB perspective on this (because you aren't going to give it to him), so just say what you feel. Not what you think about what he's doing and your fears for him. Sit with yourself.
My guess? That might be both helpful for you-- to get it down on paper-- and a better to start with what you might say to him, because I agree with your guess that the dying alone part won't really fly so well And yes, given your reaction to the conversation today, you probably aren't ready to have that conversation with him yet. But the more comfortable you are with understanding where YOU are right now and what your own fears are, the more comfortable you will be in expressing them, or not, to him eventually.
My only other concrete piece of advice... which you can absolutely take or leave... I stopped texting anything meaningful. We used to fight on text which was really stupid. Now I leave texts to business or fun with H and if I have anything important to say I call him or wait to talk in person. I just don't want the evidence of me having said it there on his text thread, plus who knows how he interprets it. Just a thought. And, you can't really interpret his responses either without the context of his voice/presence/whatever. He could have meant it along the lines of what cardinal suggests, or he could have meant holy crap I'm just so overwhelmed with feelings myself I just don't know how to respond. The only thing I know to be true right now is that I want you to know I will always, always be there for you and for D17. You know his head is a mess and he knows that at some level too. I just feel like communication is hard enough without adding in the built-in potential misunderstandings of texting.
You're so strong. You got this. Deep breath, wash your face, give yourself some slack and do something special for yourself, and get back in there.
Me (46) H (42) M:14 T:18, D9 & D11 4/19 - 12/19: series of escalating BDs 9/20 - present: R and piecing