I'd guess you're tension is really tight. If so, you need to work on loosening up some, that will be easier on your hands, wrists, arms, in fact your whole body.
I think 16" circs should work well for hats. Or you can go longer and do ML. Here's a good video on how to join your stitches. https://www.youtube.com/watch?v=9Ajp...e=results_main