zjw, I got nothing because I've been in your shoes. Jumping to fulfill unrealistic expectations unfortunately sets the bar. Then, when you're so exhausted and mind screwed you try to backtrack to show them that they're being unrealistic and they just don't get it.
I have no suggestions, you're doing exactly what I did. Eventually, they understood. Not only that but because the timeframe to complete tasks was made realistic the quality of work was better. So they backed off a bit.
The best I can offer is some humor because if you live in the same work world I do you'll laugh. Some might find this video odd but you and I both know that it's definitely not off the mark!