My girlfriend can be very difficult; she nags and intentionally starts fights. Why?

Sounds like a typical relationship to me. Just make sure that she knows tht you are going to put your foot down when you are sure of how important it is to do so. There will be ups and downs, but that is normal. If you love each other things will work out. Besides, have you timed these outbursts? Could it be PMS? If it is, just get used to it. that is how women are.