Which mental disorder is characterized by someone believing only in their own reality?

Reality: That is not one specific mental disorder. Loss of touch with reality can occur in a number of different mental health conditions.

Human State: Without being facetious, i'd say this is the state of most humans -- whether we realize it or not. Most of us are shocked when others see things differently from us. Think of all the household arguments you've ever seen; the stalemate in our congress; wars, etc. We walk around with our opinions of others, forgetting that we only see a portion of reality. In this way, we're all delusional.