hhmmmm-not sure what to say kate, because i have similar issues with my dh- except, of course, i don't have the pnd to go with it, so can't really understand how much harder that makes it on you. big hugs xoxo
could you try a day out with a gf and leave sarah with him for the whole day?
i found that because dh was working so much and such long hours in the lead up to xmas, he didn't really understand how full-on ds was becoming. since he's had a few days off and we went away with ds, he's completely shocked at how much work it is keeping up with ds. even tonight, he was dressing ds for bed while i was having a bath, and ds came running into the bathroom laughing his head off with dh chasing after him.
sometimes men are just really dumb and don't realise how tough you're doing it. after i spent the whole day picking up all his stuff from everywhere, he actually made me dinner while i sat and watched the cricket and then even gave me a back massage (it went for about 4 mins though, much like dtd with him )..sometimes i just have to absoutely 'crack it' to get this kind of treatment though.
i hope it all works out for you, like i said, a day of leaving him with dd for the whole day while you go out for lunch/movies/beauty salon could make a big difference xoxo