I just need to understand..

Hi. I’m 14, and last year my half sister went to rehab for severe anorexia. I haven’t spoken to her in over a year now, and she’s still in rehab. I just dont understand anything. I went to therapy and they told me I was ‘grieving’ her, even though she isn’t dead. I just need to understand the mindset, it’s killing me knowing I could have stopped it, I knew the signs and we had all noticed but none of us did anything. I just don’t know what to do.

This is a picture of us about a year before she went to rehab, she turned 16 just after she went in, and I was 14.