I completely understand the necessity to lie, Fire. It sucks so hard having to lie to friends and family.
My best friend is always trying to make plans and sometimes, I break down and make plans with him even though I know I will cancel the day of. I'm always making up reasons that I'm busy like, housework, laundry, fake chores and the like or sometimes I'll tell him that my Mum (who I live with) is having people over and it's not a good time. Making up excuses makes me feel like I'm betraying him. We've been friends for four years and only recently (in the past two weeks) have I explained to him why I always 'bail' at the last minute or make up excuses.
For me, it doesn't seem to matter if I explain the situation to people. Some people just don't 'get it'. They'll never get it. They will feign understanding but they'll never really know what it's like until they experience it themselves.