Ok, firstly take a deep breath. you are a good mum i'm sure, you're just feeling overwhelmed at the moment.
What you need to do is take all emotion out of the equation and the best way to do this is to have a stock answer and consequence that requires no thinking from your part.
DS1 (4) is very high maintenance, throws massive tantrums, fusses at dinner etc. i was where you are a few months ago. One evening i sat down and made a plan based around counting to 5, naughty step and reward/sticker chart.
So for things like getting dressed we do races. I challenge DS to get dressed before i've managed to get the DTs dressed, he loves this. Would your older DD be willing to help with this? This also works for getting coats and shoes on etc.
For most other things the conversation is as follows:
DS, do XXX please.
DS, if you havent done XXX by the time i count to 5 you will sit on the naughty step.
Count to 5
Naughty step.
The first few days i think he spent more time on the step than not but now he only ends up there once a day.
With things like walking i explain before we leave the house that he can walk a bit ahead as long as he stops when i say. if he doesnt stop he has to hold my hand.
Meals are very simple. I put food down, he starts fussing and i say
'Your choice is to eat your breakfast/lunch/dinner or to not eat it an be hungry. if you choose not to eat it there is nothing else until "next meal". i dont engage any further in conversations about the meal. children will not starve themselves and the odd missed meal wont hurt them.
The violence i take a harder line on, DS knows that any violence towards his siblings goes straight to the naughty step.
you need to not let the screams get to you, if she sees a reaction she'll keep going. it will be really really hard for a few days but it is so worth it, DS is so so much better now and i enjoy my time with him so much more. the key is consistancy, when DS's behaviour starts to slip i normally find its because i have slipped in the consistancy.
Can you sit down with DH and agree a plan going forward? and also try and carve out some family time? Please dont beat yourself up about it, it is all things that can be improved. I swear they get like this so that by the time they go to school we are thinking 'yes please take them!' rather than 'ooh my poor little baby!'