If someone truly believes with all of their heart that the world is flat how do you convince them otherwise?

If someone believes that they are worthless than the piece of crap you just stepped in how do you convince them otherwise? What's the point in positive affirmation if you don't believe what you're saying? What do you do if that "inner" voice just laughs and says "No you're not you worthless piece of crap"?
What's the point of therapy if it doesn't work and they don't listen?