It's definitely not you being a bad mum. I thought the same and joining COL was a lifesaver as, not only did I learn that everything I was going through was totally normal, but that I was actually doing OK. I wish I had known then what I know now and I would have stressed less and enjoyed the puppy stage more.
Pick your battles - walk away when it gets too much. Keep her busy, but not too busy and ignore people who make you feel she should be a little robot who just does as she's told. Cockers are not actually a particularly easy breed, but I still think they are the best dogs in the world.
Re other dogs, I would actively seek out some calm, older dogs for her to meet as they will tell her off when she is going too far. It is the best way for a pup to learn doggy manners. Also, the chance to go crazy and let off steam with other pups is very important at this age. She needs to meet as many other dogs as possible now - when I took Coco to puppy class, our homework was always to meet 10 new dogs!
It will get easier, so relax and don't give yourself a hard time. You are getting used to each other and it can take a while. In time you will find a middle ground that suits you all. You may never be able to train her out of certain things (with Coco it was stealing food and barking!) but you'll just learn to manage it and you will have the most loving and funny companion you have ever had