Hi Beren2016,
Validate, validate, validate.
It's counterintuitive sometimes. She says "I look horrible!" and you say "You look beautiful". And instead of listeing to your appreciation, what she gets is "he's telling me that my feelings are wrong".
Then what? Do we say : "yes honey, you certainly look horrible"? You know we don't. But you can say, "I feel ugly many times, and I know it crushes you. I wish you saw how pretty you look in my eyes".
You say it's not an eating D. It might be behing your back, there are many types and many ramifications. But the eating is only the tip of the iceberg, and one of the vissible parts, only that. Behind that, there is a bunch of identity and self esteem issues.
Adknowledge that she is having a bad time. It is hard to be validating, but it is easier to keep an eye out for not being invalidating. Avoid any statement that says "your perception is wrong". As in "people won't care" "you just need to do it" "you don't need to lose weight"... .And look for things you can validate and even reward.
Other than that, I've found we can lower their anxiety levels. Provide a distraction, or a relaxing activity. Try to avoid engaging in arguments. Be patient, don't try to change it quick.
You can say something like: "I see it is very hard for you to get out now, we can stay if you want. But if you think about something I can do to make it better for you, I'm here when you are ready."
Have you read about validation in this site?
https://bpdfamily.com/content/communication-skills-dont-be-invalidatingLet us know your thoughts and if you try a different approach. We are here to listen.