OP you have had such a tough time on here, and you've been through so much already.
It's almost impossible to see the wood for the trees once you get to this position.
I agree with those who say take baby steps.
My (probably not useful) suggestions are:
Persevere with DD in a separate room to him. It will take time but it will settle down and get easier.
GP visit for DS and one for you. You need support and your GP may be able to help.
Increase DS's nursery hours, and, if you can afford it, can DD go as well? You might then get some time to catch up on sleep etc.
I know sure start etc thought your son was too hard work, would they come back to help with your DD instead? One less child to look after at least gives you some more space in your head.
Do you have friends who can come over, either day or night? Even if they aren't interested in kids, you need some adult conversation.
I assume you have a double pram for the girls. Not sure what type of pram, but can you buy a baby carrier for the baby, and then put DS and DD in the pram. He can't run away then, which should make walking easier for you.
Hugs, you are doing a great job x