Bloss, your message is such a cry from the heart. I hope this day is better for you. You know you're not a bad mother, just an overburdened one.
On days like the one you describe, my way of coping was to get out, leave the housework, change the scene, put the baby in the pushchair propped up so they can see the world, give them something else to look at - just go. Anywhere - the park, the shops, the sand pit, the town hall, the swimming pool. Pack sandwiches and bring a bottle of water for the toddler and you, if skint.
When I was out, I knew I was letting myself and the children in for the unexpected, for new experiences, even if minor. I could put my stresses on the back burner, literally leave them at home. Couldn't worry about housework or paper work because I wasn't there to do it, and my concentratrion had to be fixed on the here and now. The route we were taking, road safety etc. The simple rhythem of walking always helped me to relax and forget.
When my oldest was a baby, he was so active and demanding. I had to transfer his demands away from me sometimes, to the wide world, so I got some peace. And for your ds, toddlers can make the same journey and every time it is different for them, as you know, of course.
Walk or drive, - do you drive, have you access to a car? This was a lifeline for me. I don't know what your area is like, are there parks in walking distance?`does the heat allow you to comfortably make family trips during the day?
That's enough of my rambling. As others have said, you are going through a very hard time and as your children get older things will get much easier.
Thinking of your you, your poor FIL and your dh.
Love tigermoth