Please call the Samaritans while you feel like this; I know you've spoken to them before.
Each day you get through is a day closer to things getting better. Your children are growing and changing all the time, and they will get easier, they will. And you will get more time to yourself, and not even feel the need for it.
You are seeing someone on Tuesday I think? Keep focused on that.
Is it possible that you would get more sleep if dd1 slept with you? This worked for me with dd2 and dd3, who came into my bed in the night until they were about 4 or 5. It was no good with dd1, just not her thing. Don't worry about rods for your back, just get through now the best way you can, and the closeness may help you both, as you may both feel you need that reassurance of having someone there.
I know you need real practical help, but you are getting through each day in some fashion, that's all you have to do just now, nothing more. I'm thinking about you, keep going, just keep going. It's always darkest before the dawn and all that... It will get brighter one day again. Just keep going. Xx