Sexual interest, arousal, and performance are all dependent on a complex interplay of brain neurotransmitters as well as both the balance and levels of male and female hormones. Diseases affecting the nervous system can directly repress sexual desire. Debilitating chronic illnesses can use up energy that would normally go towards sex. Thyroid disorders or endocrine problems can disrupt one's sex life including increasing sex drive in women with excess male hormone production or testosterone use. Sexual disinterest is also a major symptom of neurotransmitter problems such as depression.
Emotional stress is a common cause of lack of desire, especially relationship conflicts.
Once a difference in sex drive has turned into an issue in a relationship, it can take on a destructive life of its own. Try planning a positive approach to seducing your lover. I hope your girlfriend will get checked out medically, including telling her doctor the nature of the sexual problems you two are having. Couples counseling may help defuse the conflict and help you find supportive ways to work through it. Some couples have found open relationships to be a way to make sure both get enough sex.