Ditto the above, and as far as I can see you're doing a pretty good job overall.
You might want to suggest something like - ehh, I had something about offering to cancel a date with 24 hour hours notice if something important happens, or saying if you've canceled on a person once because she's asked last minute, you won't cancel a second time, but that is probably just catering to her if she is being manipulative (knowingly or subconsciously) and probably would cause other problems.
In your place I guess I would just say that these people have feelings too, and barring an emergency, you won't cancel plans with them because it would be rude and they deserve better. If you stick to that kindly but firmly, hopefully she will stop requesting you cancel.
Wish I'd had something more insightful to say, but not today