Hi Westmaple
It's tough to watch someone you care about suffer, especially when she won't accept any benefit that your support and experience could give her. My brother, usually so capable and in control, is in an overwhelmingly difficult situation that has been tearing him apart for months. I can see him spiraling downward, but, like your girlfriend, he internalizes everything and won't admit that he's struggling. I'm sure that I could help him if he'd let me, but I'm also sure that if I offer support one more time it will hurt our relationship.
Doing nothing is really hard, but their journeys are their own, not ours. I think that all we can do is to hope that they get through without crashing, and to be there for them if and when they come to us. I also think it's great that you're here -- it's helping me in particular with not taking my brother's rejection personally, and with letting go of things I can't control.
It may seem selfish, but focusing on yourself and your emotional strength is really important. As you say, at bare minimum it will distract you from worrying about your girlfriend, but keep in mind the airplane emergency guidelines as well: if the oxygen masks drop, put your own on first so that you're better able to help those around you.