Thank FUCK you still have that flat @MeanOrJustified you are going to need it love.
I get that you are being made to feel responsible, but really you are the one financing and supporting 2 family units. Your H has chosen to prioritise his DC over your family unit by not stepping up to support both family units and is expecting you and your assets to cover it all. He is totally selfish.
You are doing counselling, but i don't think this will make any difference, I think you do need to come to terms with the fact that it may be time for you to call time on this relationship.
Why? because as you say this isn't what you signed up for, and the for better and for worse was NOT only on you to fulfil, he needs to do whatever the fuck it takes to earn enough for all his kids, and not mooch off you for everything AND on top of this, pressure you to shoulder MORE debt to take kids away who would not enjoy it anyway.
I say you can offer that he goes with you and the toddler, or not at all and you will take a friend and see him when you get back. it might be the boot up the arse he needs to see you moving on.
It's a risk though, you may see how much more simple life is without him.
Whatever happens, this is not your failure, you have gone above and beyond to keep this show on the road, he has not. You need to come to terms with the next phase of your life. Yu can do this, you really can.
Work out the numbers, give notice to the tenants, give notice on your house and start getting your own life back. it will be ok.