What is wrong with people?

So I went shopping today, and tried on a skirt. You may but probably don't know I have body image issues and an eating disorder, anyways I tried on this skirt and was thinking I look pretty good in it, a thought that doesn't cross my mind often.
I go out of the dressing room to show my mum and she says
"Should we get the next size up?" not really a thing you want to hear when you have an eating disorder that you're trying to recover from, and the thing was loose, so I told her then she said the worse thing you can say to an eating disordered person
"You're butt looks huge in it, it pops out too much."
What she could've said is
"It doesn't look good."
I already hate my body, you don't have to make negative remarks on it. So now here I am sitting in my room bad thoughts in my head, which I've already been fighting with a lot, and she just makes it so much worse. My mum is one of my triggers, I know going shopping is dangerous for me, and I should've known she'd do something like that, but for some reason I keep thinking maybe she'll actually try to understand my disorder but my mum and dad like to ignore it, until I was tricked into recovery and chose to try and stick with it, but sometimes I just feel like what is the fucking point?
I'm trying this hard and they don't even try to understand.