I do the same thing. I will cancel plans with someone if i am not completely feeling up for it (most likely the thought of going somewhere at the moment, to a specific place, etc gets me anxious) then I get so apologetic to the person and tell them i'm a terrible friend, etc, and worry about what they think. Then end up feeling antisocial.

It is a vicious cycle. My anxiety cripples me to the point where I shake probably about 60% of the day, 80% of the day when I have an appointment or something important to do where I can't just lie in bed/on the couch & relax. I hate it so much. It's preventing me from living life and clouds what's truly important in life.