Going through mental problems is very hard and having to see someone you love or care about going through them can also be difficult. As someone who has gone through both sides, my strongest advice is to contact a therapist or a mental health care provider. This has helped me a lot when I was going through rough times of depression and I've seen how much it has helped others.
But first, the most important thing you must know is that you must allow your girlfriend to be willing to get the help she may need. If you force her to get help, she may not actually receive the help she needs and it would be pointless. That's why you have to persuade and encourage her to get help. This requires your patience, good listening skills, and your kindness. Allow her to see how much you care about her and that even though she won't tell her about what she's going through, you will still be willing to be by her side no matter what. Tell her how you are worried about her and that although you don't feel the pain or the suffering she's going through, seeing her going through them is painful for you. If you need advice in knowing what to say, you can talk to a therapist yourself and ask about how to help someone. I know that this may be very difficult to be patient but remember to never give up on her. It will also help to get her friends and family worried about her too. The more love and support she has, the more likely it is for her to seek the help she needs.
Anyway, I hope some of this will help you and your girlfriend. Good luck.