Girlfriend dealing with grief and depression, and I feel completely helpless

Hey there AL. This will probably be a bit long and rambling, but it's something I desperately need to get off my chest, and hopefully hear some words of advice from you lovely ladies, so here goes.

I've been seeing this amazing girl for 3+ months. We're both 18, almost 19. I'll be the first to admit that we both fell for each other very hard and very fast. I've been in relationships with women before, but never felt this way or cared about someone this much. This is her first relationship with a woman, she came out as gay about a year before we met. She's an amazing girl, beautiful, strong, and kind, and way too good for me overall. At the moment we live about an hour apart, and have plans to move in together with another roommate in a few months.

She disclosed to me very early on in our relationship that she suffered from bipolar disorder, but was on no medication for it nor did she plan to resort to medication for treatment. I've glimpses of how the disorder affects her from time to time, and always thought it would be something we could handle and work through together.

A few weeks ago, she lost her little sister to cancer. Although they knew it was terminal, they did not think she would succumb this soon or this unexpectedly. Since the loss she has not been herself, understandably so. She was with me when she found out and during the first few days after, but due to distance and busy work and school schedules we have to spend the week apart. I did my best to check in on her daily, call her at night, and be there for her as much as I could, although I couldn't(and can't) escape the feeling that I'm not there enough. The next weekend I saw her, she was despondent. Not upset even, just had a dead and defeated look in her eye. She told me that night that she wanted to die. I said that she needed to get help, but she insisted she didn't want any. She told me that the girl I fell in love with might not come back. More than once, she brought up the idea of leaving me, not "dragging me into this" because she doesn't want me to get hurt. I told her that I didn't care, that I wouldn't abandon her, and that I would be there for her no matter what because, fuck, I love her.

Since that night things have felt very odd between us. I understand that she's not herself right now, that she's grieving and may grieve for a long time. But I feel as though she is trying to push me out. Being short with me, ignoring my calls, terse responses to texts. The other night we both had off- a rare occasion- and were supposed to spend the night together. But after dinner and talking in the car about some of the things she was feeling, it got to the point where she wouldn't look me in the eye, starting crying, and said that she wanted to go. I asked if I said something to upset her, and she said no. I asked if she wanted to talk, and she said no.

Point being AL (and I apologize for this whole thing probably not making a ton of sense), I don't know what the fuck to do. I love her. That I know. But I don't know how to help her, or if I can. I feel completely and utterly helpless. I don't want to lose her, but I don't know if I can save her.

EDIT:

Thank you all very much for the kind responses. It helps a lot to have some objective viewpoints, particularly from those of you who have been on either side of something like this. Your advice is greatly appreciated. If I have time after work today, I'll try to respond individually to your comments. Thanks again AL