My daughter who is now 25 has Borderline Personality Disorder, which is only somewhat controlled by a cocktail of anti-depressants. She sounds a lot like your gf.
Symptoms of BPD
Borderline personality disorder is a mental disorder that results in four groups of symptoms:
Impaired Emotional Control: excessive, poorly regulated emotional responses, especially anger, that change rapidly;
Harmful Impulsivity: impulsive behaviors that are harmful to you or to others, such as spending sprees, excessive use of alcohol or drugs, self-injurious acts (e.g., cutting), physically aggressive acts and sexual indiscretions;
Impaired Perceptions and Reasoning: suspiciousness, misperceptions, an unstable self-image, a poor sense of your identity, and difficulty in reasoning under stress; and
Disrupted Relationships: tumultuous relationships with a person close to you that vary from extreme fear of abandonment to episodes of excessive anger and the desire to get away from that person.
See if a test like this sounds like your gf.
There are other online tests as well.
If it sounds right, get a hold of a copy of the book Stop Walking on Eggshells for information on how to deal with a Borderline person in your life. This book has helped me and my ex and his gf learn more about our daughter, what it's like in her head, and how to cope. I recommended it to a friend whose SIL also seems to be Borderline and she also thought it was great.