Thanks for getting back to me.
This doesn't really seem to be due to anything that you are doing. Your behavior seems fine and supportive and I do not think that the problem lies with you or how you treat her. You are patient, attentive, and kind from what I gather from your description of your situation.
It seems to me that your girlfriend may be suffering from depression and may need to get some therapy for it.
Here are the official criteria for depression. See if she meets some of these criteria:
Major Depressive Episode
DSM IV Criteria
A) Five (or more) of the following symptoms have been present during the same 2-week period and represent a change from previous functioning; at least one of the symptoms is either (1) depressed mood or (2) loss of interest or pleasure
Note: Do not include symptoms that are clearly due to a general medical condition, or mood-incongruent delusions or hallucinations
1) depressed mood most of the day, nearly every day, as indicated by either subjective report (e.g., feels sad or empty) or observation made by others (e.g., appears tearful). Note: In children and adolescents, can be irritable mood.
2) markedly diminished interest or pleasure in all, or almost all, activities most of the day, nearly every day (as indicated by either subjective account or observation made by others)
3) significant weight loss when not dieting or weight gain (e.g., a change of more than 5% of body weight in a month), or decrease or increase in appetite nearly every day. Note: In children, consider failure to make expected weight gains.
4) insomnia or hypersomnia nearly every day
5) psychomotor agitation or retardation nearly every day (observable by others, not merely subjective feelings of restlessness or being slowed down)
6) fatigue or loss of energy nearly every day
7) feelings of worthlessness or excessive or inappropriate guilt (which may be delusional) nearly every day (not merely self-reproach or guilt about being sick)
8) diminished ability to think or concentrate, or indecisiveness, nearly every day (either by subjective account or as observed by others)
9) recurrent thoughts of death (not just fear of dying), recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide
B) The symptoms do not meet criteria for a Mixed Episode
C) The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.
D) The symptoms are not due to the direct physiological effects of a substance (e.g., a drug of abuse, a medication) or a general medical condition (e.g., hypothyroidism)
E) The symptoms are not better accounted for by Bereavement, i.e., after the loss of a loved one, the symptoms persist for longer than 2 months or are characterized by marked functional impairment, morbid preoccupation with worthlessness, suicidal ideation, psychotic symptoms, or psychomotor retardation.
If you feel she has depression (and it seems quite possible) you can get her this professional workbook, particularly if she is hesitant about seeming a counselor or therapist.
I wish you great success.