Oh bless you, I've been pretty much where you were, except for the c section! I have no advice there, but please take it as easy as you can.
DD was 4 when DS was born, and nearing the end of preschool. DH had 2 weeks of paternity leave, and had to go back to the office on the first day of the summer holidays... Great timing! I remember one night, about a week in, sobbing in the kitchen about how I was never going to cope, DD would never forgive me, I wouldn't be able to give DS special attention, etc. It was my lowest moment in years and I found myself regretting ever having DS, which made me feel worse.
However, I did cope. DD started school when DS was a few weeks old, so she got a break from him and to feel like a big girl, and I got to spend time with him on my own like I did with DD. DH and I made a conscious effort to both spend alone time with DD, even something simple like taking turns to read to her before bed (we still do this now and she's 8). Four years later and it's tough sometimes, but we're all still here, nobody's been left out or forgotten, everyone is fed and loved. DS is starting school in Sept. It whizzes by, truly it does.
In terms of practical advice, the big one is to pick your battles. Some things really really don't matter, so save your energy for the ones that do. Does your DS want to help with the baby? DD took (takes!) her role of big sister extremely seriously, and was very involved with things like fetching nappies, waggling toys in his face, reading stories, etc. It made her feel important, like we were a team trying to keep this tiny human being alive together.
The first few weeks can be rough, OP, I won't deny. But it WILL sort itself out, you'll find a rhythm that works for all of you, and after awhile you'll wonder what you were worried about. Sorry for the long post, but I sympathise so so much 