When my H kept talking to me about his issues, what was going on in his head, etc., labug reminded me that all of the sharing was about him getting his needs met and had nothing to do with my needs, and to gently remind him that he can discuss these things with his IC, that's why he has one. If your H is still seeing an IC (I think you said in the past he was, maybe?) that may be a way to handle it. But I don't think you need to force yourself to sit and listen every time he wants to unload just because there's so much talk about validation and listening on here!
Re: the separation stuff, it's hard to say much without knowing about the legalities where you live. If he's the one to file can you still receive public assistance? Ultimately, if you have to do something you didn't want to do to ensure you and your D are protected financially, it may help to acknowledge that while you didn't want this and don't agree with it, you are accepting it, and accepting it may mean you'll have to initiate certain things in order to handle the situation. I was given similar advice by many when I was thinking of moving - I didn't want to be the to have to make the effort and initiate the physical S but it became part of accepting the situation and what H was telling me at the time (that he was 99% sure that D would be the end result of all this.)
Me:30 H:29, no kids T:12, M:4 (when D was final) 12/13: "Don't think I want to be M anymore" 6/14: Separated (I move) 1/15: H filed for D 5/15: D final