Recently, a friend invited me to an event that was important to her.
I said "I have an appointment." but worry I may have blown her off. I didn't want to go into specifics - the appointment was for beauty (nails, etc).
This is more important to me than going to her event. But if I'm forthcoming about the type of appointment she may think it's trivial compared to the event she was inviting me to. And I didn't want to instigate a longer conversation about my priorities.
In the future, how can I say something like "I have a prior commitment" or "I've had a long-standing appointment; sorry I can't make it" without sounding passive-aggressive or rude or overly formal?