First thing I learnt, and I don't know that much, is that the sub can always say no. There are two phrases used to describe conduct - SSC and RACK. In both the C is consensual. If you don't consent it shouldn't happen.

It is also normal for a Dom to push you, to help you to develop yourself, but if there is something you really don't want to do you shouldn't just because your Dom says so. You should be able to talk to your Dom, discuss it and come to an agreement. If He isn't willing to listen to you and work with you He doesn't deserve the capitals I have given Him.