A new self-discipline is always difficult to start. Take diet, for instance. You want to eat less sugar, maybe ice cream is the vice. The first day you vow not to have any, and your mind goes into a panic.
Immediately, the worst-case scenario comes to mind. "There's no way I can go the rest of my life without ice cream!!" Of course, the truth is you'll probably have ice cream again sometime, just hopefully not in the next ten minutes.
But if you weren't trying to avoid ice cream, you probably wouldn't be panicking about it. Yes, there are physical challenges to changing disciplines, but the mental ones are even greater.