You usually use "this" when referring to something that's nearby or that they are holding. People use "that" when referring to something that's a distance away, but still insight. "I can't get that far" is a statement someone would probably say while pointing to a certain spot. "I can't get this far" is a statement a child would probably say using his hands to measure and try to communicate with you a certain distance that he/she can't reach. "That test was that difficult?!" Is something someone would say either because they're surprised how hard the test was, or because they're being sarcastic and the test you failed was really easy for them. "The test was this difficult" would be when someone tries to explain how difficult the test was to a certain measure.