I believe I can help you.
Your girlfriend is experiencing emotional preoccupation that is the result of trauma. When we are involved with high anxiety/emotional situations our brains attempt to cement the experience into long term memory. This is a survival mechanism that is useful to recall experiences that are a part of our control. But, when the circumstances are not, and it is the results of abuse, the emotions and memories stay with us, often in a dysfunctional way. This is the same mechanism that creates Post Traumatic Stress Disorder and although I do not believe your girlfriend has the full disorder, she is showing many of the traits.
What should you do? Trauma of this nature is causing her to feel inaccurate feelings. She is not longer in love with him, for certain, but the emotional memories are still present, causing her to dwell of the feelings and thoughts of this relationship.
Fortunately this is treatable and medications are not the total answer, although they can be of some help in the short run. Instead, she needs to see a counselor who is familiar with trauma based treatments such as EMDR and similar. These techniques are used to desensitize the events and lessen the impact of the trauma. The techniques are safe and effective and many counselors are trained in the process. Your family doctor should be able to refer you to a CBT (cognitive behavioral therapist) who will either practice this or know those who do.
In the meantime I would highly suggest an excellent and inexpensive resource: The PTSD Workbook: Simple, Effective Techniques for Overcoming Traumatic Stress Symptoms by Mary Beth Williams and Soili Poijula (Paperback - May 15, 2002)
This book will open up a great many options for your girlfriend and help her to reprocess the feelings she is unable to resolve. The book is available on amazon.com
With trauma oriented counseling and the book I recommended, she will be well on her way to getting over this problem. Steven