Hello,
You can help anyone only if they are willing to take help, if she is not willing to communicate with you then leave her alone specifically 'cause she feels you are pulling her back to unpleasant(bad) feelings. Let her stay comfortable.
Since you say you think about her constantly, assess yourself, maybe you need her. She looks to be a little fickle minded since she differs (from talking about
depression to being in a good place) from what she says.
She ought to stay away from addictions if any, they do more harm than good. To get depression treated she should visit a
psychiatrist or a counselor, sometimes
talk therapy helps but patients may need medicines.
She needs to be physically examined & assessed. Her treatment will depend upon the severity of the problem detected.
It's good if you want to see her happy but avoid getting pulled into her problems, it may get you crazy or restless & interfere with your daily activities(studies /work, etc). Yes, do help her if she needs any kind of help.
Hope I have answered your query. Let me know if I can assist you further.
Take care
Regards,
Dr Nupur K, General & Family Physician