You're not doing anything wrong. This is SO normal for her age. She will grow out of it, and in the meantime you just need to concentrate on managing it as best you can. I well remember how absolutely infuriating it is to have a puppy hanging off your leg.
If saying 'ouch' or yelping makes her worse, stop doing it. I know it's generally the advice given, but I think exciteable little pups like Cockers just feed on the energy. In fact that's something I learned early on with Henry - energy in = energy out. Keeping my voice and movements very, very calm did help calm him down.
Ignoring her and separating yourself from her isn't really about making her 'bothered', more about giving yourself time to cool off!
Rather than playing ball, can you get a long tuggy toy that you can also fling? So then she can have a good game of tug when she brings it back, while keeping your hands safe.
One final thing: I remember with Henry that behaviours would be at their very worst just before a big developmental leap. So many neurons firing off in their little brains. So a bad day doesn't necessarily mean a step backwards. It can mean a big step forwards is just around the corner.