@PithyBeaker Glad you had a nice time away. Sorry the end of your holiday was tainted by this selfish manchild.
I once saw a therapist who gave me a great technique for dealing with this kind of situation. Mine wasn't a breakup, but it had a similar lingering effect. She told me to think about the mantra they teach people at AA.
Apparently, they tell recovering alcoholics to "think the drink through." It's a really effective way of bypassing that moment when the brain is clamouring for its fix and telling itself that everything will be okay if they can just get the thing their brain believes it needs. Instead, they have to sit for a few moments and think slowly and logically about what will happen the second after they take that drink, the minute after they take it, the hour after they take it etc.
In your case, I would advise you to do something similar. Sit for a while each day and "think the drink through". Imagine that you are at a very low point, missing your ex and wondering whether you have done the right thing. You feel as though you have miserably and permanently screwed up your life and made a massive mistake.
That's fine. Give yourself mental permission to take that first drink ie: call him and relieve that immediate pain by telling him you will get back together with a view to him and his children moving back in in a few months' time. Allow yourself to feel the lovely relaxing emotion as your brain gets that instant relief from the painful feelings it is currently experiencing.
Stay in that feeling and start thinking the drink through in small chunks of time, while listening very carefully to your body and what it is saying to you. Imagine his car pulling up, and him and his children getting out with all their luggage. They walk up your garden path, and you open the door to them. They burst inside, race upstairs to the rooms you have had to clear out for them, dump all their stuff, and start fighting.
Where are you feeling that in your body? How do you feel when you realise there is no going back from this one? You can't throw them out again without a massive amount of upheaval and pain, so you and your ex both know that you are pretty much stuck with it, no matter how badly things slip.
How about your ex? What is he doing while his children rampage about the house? No doubt he'll be telling you how happy he is that you have finally come to your senses and allowed them back into your space. There will be a certain amount of martyish reproach in his expression as he offers to switch the kettle on and make you a cup of coffee. But, being the good man he is, he's prepared to overlook it this time, as long as you don't behave in his words like "a cunt" again.
Move forward to your son coming home from his father's house or school and walking into that previously peaceful, clean, ordered environment and realising you have again allowed to be invaded by this man and his sons and there's absolutely nothing he can do about it.
Do that often enough, and you will be able to conjure up that physical feeling in seconds, rather than having to think it through step by step. Hold on to that feeling and internalise it. Every single time this man contacts you with a demand, a fresh argument, or a plea, go straight to that place and allow your body to feel all those sensations again. Every time you are sitting alone in the evening and wondering whether you have stuffed up your life, go to the same place and feel the same feelings.
I would honestly be massively surprised if, after doing this for a while, you don't feel an instinctive sense of revulsion each time he either suggests or you consider continuing a relationship with them all. On the other hand, a year from now, your life could be in tatters again and your mental health seriously affected simply because you failed to "think the drink through".