Hi C.K., I'd like to help you with your question.
I agree with you, it does sound like your girlfriend has depression. And I think your assessment of the reasons are good as well. She may be feeling that there are too many situations in her life that are negative, and she has taken them on as meaning there is something wrong with her.
Sometimes people experience depression when they feel overwhelmed by life's stressors. When they do not feel they have the resources to cope, they become depressed. Your girlfriend may need some help to feel better.
Talking to her is great. It helps her know she is not alone and that you care. Support like that makes a big difference.
You could also talk to her about seeing a therapist. She could have an evaluation done to determine if she does have depression and what type (it ranges from adjustment disorder to severe depression). The therapist can then offer her the appropriate treatment for her diagnosis. Most likely, given what you have described, she would probably benefit from short term outpatient therapy. It would help her express her feelings and learn new ways to cope.
She can also help herself. Here are some resources to get her (and you) started:
The Depression Cure: The 6-Step Program to Beat Depression without Drugs by Stephen S. Ilardi
Undoing Depression: What Therapy Doesn't Teach You and Medication Can't Give You by Richard O'Connor
Talking to Depression: Simple Ways To Connect When Someone In Your Life Is Depressed: Simple Ways To Connect When Someone In Your Life Is Depressed by Claudia J. Strauss and Martha Manning
You can find these books on Amazon.com or your local library may have them for you.
I hope this has helped,