My son, didn't want to part with either, and at times had to be forcible held on the potty. He would not go and was in obvious distress as he needed to go, would not put on a nappy, but would not go on the potty.
Eventually I had to forcibly hold him on the potty,(this was utterly heartbreaking) but as soo nas he started, he relaxed and started to smile!!. I think that it was a fear thing.
I had to do this twice, (and believe me I felt like the most awfu mother in the world), theresfter sitting reading him a story while he did it worked.
I don't know whether this helps, becuase it is a horrible thing to do, and I only mananged because my mother was there and she said that I was exactly the same.