Except that you can, by just doing the things you want to do until they become habits. It's not really much different than going to the gym or deciding not to drink too often. Even though you might want/not want to do the thing, you still do it because you know it's better for you.
The better way to say it is that it's difficult to alter one's desires via reflection, not that it cannot be done.
The better way to say it is that it's difficult to alter one's desires via reflection, not that it cannot be done.