Ok, deep breath! I remember going back full time after my son and it is hard at first, but it does get better and it is manageable 
Sorry that your daughter has had d&v. They do tend to pick stuff up when they first start. However, she should be ok to start in 2 weeks? Do you have any family help? A partner/parents? If so, can you plan with them who will do what if dd is ill and can't go to nursery?
In the first 6 months of nursery, my son had a lot of ear infections. DH and I shared the load of time off but my mum also helped a lot which I will be eternally grateful for!
With the prep for school, break it down into very small chunks. What do you absolutely have to get done? Break those tasks down and do those. Everything else can wait. One thing I had to accept when I first went back is that sometimes, things just have to be good enough. You no longer have time to spend ages faffing about making a fancy PowerPoint or putting nice borders on a worksheet. It needs to be functional enough so the kids learn and no more. Look on TES/twinkl for resources and use those. I'm not primary so can't offer resources but maybe others can help with this.
Do as much as possible in school time. Use your lunches/PPA/any time you can have before/after school while nursery is open to do your work. Less to bring home that way.
At home - lower your standards a bit at first. Simple evening meals, basic cleaning and make sure yours and DD clothes are ready each night. I organise all of ours on a Sunday for the week.
Do you have a trusted colleague you can talk to for a bit of support? Who taught the class last year? Can they offer advice on what worked?
It's hard at first but you will get into the routine of it. I found the Autumn/Christmas term the most difficult and then it started to get better.