I think you're probably knitting really tightly. If you can try to make your stitches looser then that might solve the problem. They should slide easily on the needle. You chose a good cast on. If you don't make it too tight it's easy to work the first row with the long tail cast on and it looks good. Don't get discouraged, we've all had to learn the tension thing. I used to have a problem with my long tail cast on being too tight and until I got better at it would actually use a needle a size or two larger than I planned to work with. HTH
I should never overestimate my basic arithmetic skills.