Rape is such a horrifying thing. The effects stretch on long after the act itself.

Even if you didn't know her when it happened, you want to protect her and wipe it away. You're enraged at the man who violated her. Even if she has been ok when she is sexual with you, it is hard to forget she has gone through that type of violence.

You have every right to feel this way. There is nothing "wrong" with you. I would encourage you to slow down and make "better memories" or images to think of. Everytime you think about her "details", replace them with an enjoyable time you two were "together". It will be awkward at first. Keep trying. Eventually, you'll find you can put it out of your mind.

Not absolutely sure that will work, but it is worth a try. Whenever I have had issues with having been attacked, it's been hard. I know it's been hard for every male (husband, best friend, father) in my life to have comprehended when they found out. Hang in there!