First, the taking tips from the sidebar are a good place to go.
Second, this
I've tried to talk to her and get her to see that there's still hope and she has people that love and and will do anything to help her,
Helps less than you think. When you're seriously suicidal in the kind of way that your girlfriend is....and there's more than one kind.....someone telling you "No, you're wrong, there really are good things about you and people that love you", just isn't something we believe. Or even if we do, it doesn't really make a difference.
You're goal shouldn't be to change her mind. It's not about making her think differently or getting her to see that "We really do care, tehre really is hope, life is rainbows and sunshine." Your job is to be there for her, and support her, not to argue that things really will be okay, or convince her that her thoughts aren't correct. Someone supporting you and not trying to change your mind helps infinately more than someone trying to show you that your perspective is "all wrong", or that things really are wonderful when you know they aren't.
If anyone can give me any advice on what to tell her? I would be so grateful to you.
See, the problem is you don't need to "tell her" anything, except "i love you, and i'm not going to pressure you into being or thinking or feeling differently than you are." If your goal is "I have to convince her that she's wrong, that life is worth living, that people really do care, that all her problems are solvable because I think so"....then you're just going to make her feel more alone. I have been on the receiving end of all of that, and all it did was make me withdraw from anyone who kept trying to "fix" me.
I love her so much and I can't stand the thought of losing her and her feeling this way
You cannot control how she feels....part of your relationship, at least for now, may be you accepting that you cannot make her feel happy all the time, or make her feel better, or change her thoughts. You have to accept that all you can do is be there for her and support her. The only friend that ever helped my suicidal thoughts (they didn't go away, but this friend helped them for a while) was the only one who never tried to change how i felt or thought. They were just there with me, hung out, hugged, things like that.
In terms of treatment, you're not going to find something that works until she feels hopeful enough to try it. Right now, she doesn't want to try or see the point in it, and you could suggest a hundred treatment options, but she might not take any of them, because she may not be in a place where her goal is to get better. So your focus needs to be on "i support you and whatever decision you make in this thing, and i'm not going to try and 'fix' you or change you. I'm just going to be here for you throughout.''