You can't control others, but you can control yourself. You know that a day without eating is a wrong behavior. Even if your emotional pulls to eat nothing are strong, you must get reasonable nutrition while losing weight.

You obviously are very educated about this. Pull on that wisdom and education to override your emotions that tend to poorly lead you. Your emotions might feel anger, rebellion/defiance, low self-esteem or worthlessness -- you don't have to assent to any of those thoughts when you have them. Don't grasp them or give them consideration, just let them float by.

The serenity prayer says we should accept the things we cannot change. That's for serenity, our peace of mind. It won't make a sucky situation better, but it'll help us endure through it better if we aren't complaining about it any longer or grinding on how unfair it is.

My SO didn’t back me in this appointment. He just gets mad. He said nothing to her. He said I was crying for nothing.

Do you trust your SO's judgment, generally? If so, consider whether you should trust it here, too. After all, your SO is more objective than you could be about something so raw and triggering to you. That doesn't mean your SO will always be right, but they might be righter than you will be since it's very personal and emotional to you.