People sometimes seem to imply that if we could just select the most appropriate types of language, and only express our true, heartfelt feelings, then our language will never cause pain.
But that's just not true. Sometimes your friend may also secretly want to cancel, but other times your friend will be hurt by knowing you want to cancel the plans you made together that they have, for whatever reason, been really looking forward to. And sometimes there doesn't exist a way to communicate your true feelings without potentially causing pain. Being mature and communicating truthfully cannot solve this problem. Often times the solution is to suck it up and stick to the plans -- but that has nothing to do with communication.