Reading your account of thing I do not think you have anything to reproach yourself about, neither your views nor your reaction. I know it can be very easy to doubt yourself in the face of opposition but here I'm pretty sure it is not warranted.
If you look at this with perspective there are several important points, the first being that this person has been a disruptive influence in the past and you had tried to go along for the sake of family harmony. There are however limits.
For anyone to condone abuse of staff - or anyone else for that matter - is really rather reprehensible and unfeeling. From your own experience as a waitress you know how bad it can be, so to take an opposite view is quite OK, particularly if you know your own past and efforts are being belittled, as was the case here.
Both your MIL and husband agree with you, which is further confirmation you acted reasonably.
If you then remember you do have an anxiety condition, and there will be topics or situations or interactions with people you will find make your anxiety worse, then this has simply been one of them
It looks like it has started your anxiety over your own behavior, something many feel - myself included - even though it is not something you need to worry about (yes I know, pointing out the logic of all this is only a small help).
May I ask if you have sought medical resistance for your anxiety, the reason I'm asking is I simply got worse until I had competent medical support.
Can I suggest that you try to put the incident out of your mind? Many people have strategies for dealing wiht excessive anxiety, and one becomes more adept with it over time.
Why not have a look at
It is awful long but I've found may treasures in it I've adopted and they have helped. I use exercise, books, movies, pets, talking with my partner and other things too. I'd expect you will have a grab-bag of ideas to help you.
Please do come back and talk more