LOTI - I see where you are coming from and I see the pain and fury and I know you have not had enough sleep. I completely agree that you should not be a doormat, or a martyr, or the housekeeper, and you do seem to have been left on your own a lot and you do seem to bear most to all of the responsibility for looking after the girls, and you feel tired and lonely a lot of the time which is not the way to feel in a loving partnership.
From the outside, though, it seems to me as if you and DH have not really had time to address what's been going on over the last while. You yourself said some months ago that during the first 6 months or so of dd2's life he'd not been great, but that you had worked at it and things improved dramatically - they have slipped back dramatically now again, of course. I know you said you had a chat a week or so ago, but did anything really sink in, did he understand where you were coming from, how desperate you were becoming, and was he really concerned about the state of the relationship?
I am never in favour of anyone staying in the wrong relationship but consider again, as I'm sure you already have: these issues could never be worked out without the pair of you agreeing to give them airtime. You have been becoming more unwell over the last while, you've been tired, fed up, feeling low, left behind and lonely. Your dh seems to have carried on regardless, plenty of social engagements outside the family, work and social life coming first. Would it be worth, before making the final decision to be apart, going to talk to someone else, a neutral third party? Discussing it with your dh when neither of you is tired, when both of you can give it a little time, making sure that both of you are interested in seeing if it can work, and having that hour or two hours a week, a ringfenced time when you have no other commitment than being in a room together with a calm counsellor.
If you are convinced this is the right thing to do I don't want to interfere with your courage and your resolve, and I'll be right behind ya, sister. But would it be worth trying the counselling route? I suppose if dh won't even go there perhaps it indicates that he's not really invested in the relationship.
It just seems astonishing that someone could be asleep on a sofa at 6am having been told at 3am to get their own place and leave their wife and children. If I were him I'd have been either on my knees beside your bed, or face down in a drawerful of my children's vests inhaling their scent!
LOTI - do what you have to do, don't allow yourself to be the doormat, you must maintain the integrity of your own character, you must maintain your dignity, you must be free to be happy. You are entitled to be happy, and to pass that happiness on to your girls.