How to deal with friends who have mental health issues when ur also struggling

So, my friend Brooklyn has been struggling with clinical depression and an eating disorder. I try my best to support her and be there for her when she needs me, but even I have my own shortcomings. I’ve been struggling with severe bod dysmorphia and my recently diagnosed BPD and ADHD, but I haven’t bothered telling her because she doesn’t need the extra stress. I have nobody to talk to about my problems and it feels like I’m choking on it. 

I want to support her, believe me. She’s one of my closest friends and I couldn’t imagine life without her, but the friendship feels deeply on sided. I try to save her from falling off this cliff of her own life that I slip in the process, but nobody’s there to catch me. So I’m kind of just stuck. Stuck feeling like I’m failing as a friend and that I’m so selfish for needing support when she needs it more.

Anytime I’m going through something she’s breaking down and on the verge of going into a mental hospital, so I have to push everything down to make her happy. Make her feel better. But I don’t feel better.

Then I’ll take time for myself, and I feel selfish because where does that leave HER?

I don’t know what to do