it is your brain trying to make sense of it and failing, it is able to process it happened, but not the why or how to make it better. talk to your doc or pdoc if you have one, a rape councillor should be able to help you through this difficult time.
in the meantime try to think of other things as you fall asleep, what worked for me was thinking up a story about going on holiday, the tiny details of the colour of the sea, the smell coming from the bar i passed on the way to the hotel etc, all fictional, but in enough detail to distract my brain from the nightmares. on the occasions when the nightmare came i would wake up and conciously think about the story again till i fell asleep, after a while i managed to do this without waking up until eventually i woke up one morning not remembering even having the nightmare that night.