Teenager with anxiety/depression

Hi Mummas, I’m wondering if anyone here has experience with their teenager suffering from anxiety and depression? I need some help.. but not for them, for ME. I feel so lost, exhausted and hopeless. My child has an anxiety related eating disorder. They are receiving treatment and are doing well physically, it’s the mental component that’s proving more difficult to contend with. I have given my all for the 18 months this has been happening but it has really taken a toll on my own mental health. I am usually a positive, happy person but I feel this weight in my stomach and on my chest that is starting to become a constant. My poor child is so negative and pessimistic, refuses to look for or see any positives or hope and it is exhausting. They hate change or any deviation to their usual routines or when things don’t go their way. When these changes come up, as change does in life, they get upset, angry and even hit themselves. It’s so distressing. I try to reason with them and point out other perspectives and why these situations are good but they just refute everything I say or shoot down what I suggest to help. Sometimes I feel like they’re happier being miserable. And when things are really low I seem to bear the brunt of their sadness, anger or low moods as I know they feel safe to direct it at me because I’m the mum and I’ll love them anyway, but I feel a metaphorical punching bag. I feel like I have almost nothing left for my other child.
And I feel so guilty for having these feelings because it’s not their fault they’re like this, it’s the illness.
What can I do to help myself help them? Can anyone suggest some something that they found that helped? I am losing who I am and I miss when things were normal, when we were all happy. This mental illness is ruining our lives. Any suggestions given with kindness are most welcome, especially ones coming from experience.
Sorry for the long post that’s all over the place and thank you in advance x