I am wondering if the length on the fixed circulars may be an issue as far as finger and hand positioning are concerned. I have found that the interchangeable needles are longer in length than the fixed circular. I have a chiaogoo interchangeable set of needles and a cable both of which I was able to purchase separately, avoiding the cost of buying an entire set. You could try purchasing a set when you are ready or perhaps someone you know has a pair or interchangeables you could try using.
When you are healed you might consider finishing your blanket and then start tackling this problem again. Which part of your fingers/hand are affected?
I'm sorry you are having to go through this, not only the pain but the frustration.
LunadelaOssa on ravelry.com