Please help i'm genuinely worried.

My friend has been depressed for about 2 years or so. She doesn't act like she is. On the contrary, she acts like one of the brightest people in the world. She's kind and helps other people with their problems. She's also good at drawing, or was. She stopped drawing when she began acting differently. I keep trying to ask her, but she won't open up. I've waited for her to tell me on her own time, but recently I saw in a notebook of hers that she planned on killing herself at the age of 18. I'm honestly panicking because she's 15. What if she does kill herself? Who is going to be by my side? I also realized recently that I have feelings for her. (right out of a yuri manga, I know) But, jokes aside. How can I help? And can I EVEN help? She doesn't have mangago so she won't see this post.