Plus size problems!

For the orchestral concert on the 28th of October, we were told to wear long black dresses, without showing any knee. At first, being a girl, I was quite excited at the prospect of potentially getting another dress to add to the collection.
Today though, I have had the age old problem of sizeism in shops. It is so difficult to find a maxi dress at this time of the year, let alone all black and plus size!
It seems that people don't understand that being fat is equally as much of a psychological problem as being anorexic or bulimic. Shops still stock size 4-6, but not plus size. (Not saying that all people who are sizes 4-6 have a problem) You should be treated exactly the same as someone with anorexia or bulimia. It is first and foremost a psychological problem that is using food as a trigger. It is not a sign of being a lazy slob, and the NHS patronising people with: "food has these things called calories in them" does not help the situation.
How is making shopping difficult, making it a demoralizing experience helping them? How is that giving them more self worth?
I have lost nearly half a stone so far, but I wonder what the point is. I am still landed with the fact I can't wear the pretty clothes other girls wear. Something needs to change, and I hope and pray it will come soon. x

(Btw please let me know what you think of this dress, would appreciate the comments)