Hey there, I also have a DD who has just turned two, and 6 month old twins (am bf too), so I know exactly how busy it can be.
It sounds as if you have been coping amazingly well, given that the bulk of the childcare seems to be falling to you. I am quite in awe of you, cos I need thankfully have lots of help (family & friends nearby)!
I would second what everyone else says about getting some you time. It's hard not to get caught up just being a mummy; sometimes it's so difficult to get the time as well (DH & I have agreed that in the very near future I can return to the gym, and he'll be in charge of the kids while I'm away).
People kept offering me help; I stubbornly refused at the start, but DH works long days/nights, and I definitely need help getting 3 kids ready for bed. Do take people up on whatever they offer to do; they shouldn't offer if they don't mean it!
Have your DTs started spoonfeeds yet? Mine have & at least I know I have a window when someone else other than me can feed them.
As for housework, do what you can. I have accepted that my house will never be immaculate, & for the foreseeable future there will be toys strewn everywhere, spew stains on the sofa & babyrice/Cheerios on the kitchen floor
I find it difficult not to lose my temper with DD too sometimes. I know I expect too much of her sometimes; she's only little. I've tried counting to ten (it sometimes works!) and I'm trying to pick my battles (i.e. don't sweat the small stuff!). But it's hard, isn't it? Like you, I find it easier when we get out of the house.
Hope this is not out of line... when I'm not on mat leave, I work in mental health. If you have had an episode of depression before, then yes, you'll be more likely to have PND than someone who has never been depressed. But you're absolutely right, it might just be that you are shattered! I'd go along to your GP & have a chat. There are lots if treatment options if it turns out you are depressed. If you do need ADs, Sertraline in particular is safe to take whilst bf.
Hope some of that has been of some use! Sorry about the long post!
Wishing you all the very best (& remember, you are doing an amazing job!)