Thanks for the replies, everyone. Sorry for not posting for a while, busy and tired and at the end of my tether.
So, update...we had a big discussion/argument on Sunday night after DH came home. More of the same that it's always been since time immemorial, really, but this time, instead of thoughts of separation just drifting through my mind and me dismissing them, I was genuinely considering it. Properly, seriously. First time ever.
(Actually because of mombie. You actually gave me my first glimmer of hope. Seriously. If I could somehow afford someone to help me, to have someone there to help out (and have childcare/clean home/somebody around) I could cope. Not sure if that could happen financially (I'm only on about £10k before tax credits etc) but it's something to work towards, hey...)
I also cried for about 2 hours. 
Anyway, the next morning I had a couple of hours spare and we actually talked about something IMPORTANT during daylight hours for a change. I told him properly about considering divorce - that I really had considered it genuinely and practically for the first time ever. I said if he wasn't careful he was going to lose me. He actually said, "I know." So I answered, "well, maybe it's time to be a bit more careful, then."
Since then he's been trying hard, but whether this will last, I don't know. I can't cope with it forever. I love him and want to be with him, and most importantly I don't want my kids to have a 'broken home,' but only time will tell whether things will indeed change. What usually happens after an argument is he's nice to me for a day and then is back to his selfish default. That's all it is, too - nice. My special treatment is that he is what I would normally expect from a relationship...
We're on 2 days of 'trying' so far... But I can't carry on like this, and maybe, just maybe, he's finally seen that.
I am mostly expecting to still be let down, but a girl can dream.