TW: self-harm, suicide

i'm starting to struggle with comforting my friend.
I've been doing this for a while, but recently it's just all been a mess.
She's depressed.
She self-harmed.
She keeps insulting herself.
She won't stop being so...negative.
I don't feel like any advice I give her is helping.
Everything I say feels like nonsense.
Why?
Because I don't understand!
I don't exactly understand depression!
I understand what it's like to want to self harm at one point in life, but I don't understand what it's like to actually go through with it!
I want to tell her to not vent as much beside it's stressing me out, but I don't wanna make her sad!
Even worse, she might take it the wrong way and get really upset because she thinks her friend hates her and then she might commit suicde!!!
I DON'T WANT TO LOSE HER!!!!!
...
i wonder if she'll see this.
If so, I'm sorry.
I'm sorry i can't handle the circumstances, but I'm trying.
I really am.
Just don't leave.
I don't want to lose a friend...