Oh lovely. You poor thing. I had PND with my first and all the things you're thinking, I thought too. I googled adoption, I considered running away, I went to bed every night and hoped that one of us died in our sleep. It was awful. But fluoxetine saved my life and things are really different now.
Through the whole thing I was convinced I would be the one person who didn't get over PND and didn't love my child, no matter what everyone promised me about how I would feel better and feel that bond. But I did and you will too, I promise.
Your son is too little to appreciate having a brother just yet (and newborns aren't much fun!) but he will, in time, and that blessing will by far outweigh the temporary discomfort he's experiencing from losing your full and undivided attention.
Take your sertraline, remind yourself that you got through this once before and you will again, and take things an hour at a time. You've got this, I promise.
Other practical advice: ask your HV or GP to refer you to HomeStart - they're a wonderful organisation and they will help. Get your DH to take your DS2 for a couple of hours at the weekend so you can have special time with DS1. Focus on DS2 when DS1 is at school but allow him to just fit into your routine with DS1 when he's at home as much as possible - this will get easier as he gets a bit bigger. Let DS1 "help" you with DS2 as much as possible - praise him for being such a wonderful big brother etc etc. Find something special you can do with DS2 that's just yours and just for fun - I did massage with DD and it was a really good bonding exercise. And ask for help. Ask everyone you know. If you were one of my friends I would be there in an instant. You'll be surprised at how many of your friends will have experienced what you're going through and want to help.
PM me any time, lovely. 