I have a crush on a depressed girl?

So there's a girl I'm friends with, and recently she's become extremely depressed and suicidal at times. She's attempted self-harm several times and I'm there for her every day, supporting her as she goes through this.
She tends to have these depressive states in bouts so they come and go very suddenly, but when she's fine, she's wonderful. We have so much in common, we're both open about everything and we even sorta flirt a bit. But when it comes back to her feeling really low about herself, I feel as if there's nothing I can do anymore considering I already do as much as possible.
It also makes me feel helpless as I can't start any sort of relationship with her but we're still so close, what am I supposed to do? I feel like shit for not letting her how I feel but I don't wanna burden her with anymore emotional attachments than she already has stressing her out.
I have a crush on a depressed girl?
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