Pheebe
Not sure I have a lot to say other than as you do sound rather down at the moment, so I wanted to at least post something...
Your post struck a chord with me as I remember feeling exactly the same when DS1 was about 5-6 months old.
I think about this time is when you realise properly that your life has changed forever, that the baby is here to stay, and yet you haven't really fully readjusted to it - I remember getting horribly frustrated, and that sense of 'never getting anything done'.
I'm afraid my only real advice, if you possibly can, is to do only what absolutely HAS to be done and try to stop worrying about the rest.
Are you getting out and about enough - do you have a group of friends with similar aged babies? Certainly I don't know how I would have got through this stage without having something to 'aim for' each day - even if it was just a walk in the park/ coffee at someone's house etc.
Try not to be too hard on yourself about how you look too - saggy tummies do return more to normal but give yourself time - it took 9 months to get like that, so give it at least the same time to recover .
If, as you say " Most days I'm fine and enjoy life and am really happy" then it sounds as if you're doing really well. But when you've had a poor night's sleep/ felt irritable or tired it's completely normal to feel a bit low, so you're certainly not alone.
Mo2
xxx