Paranoid Girlfriend?

I have been with my girl for nearly three years and she is still paranoid when it comes to my faithfulness. She is my third girlfriend (third one should be the charm!) I dont think about being with other girls (or guys). Whenever she has a dream about me cheating on her, she gets so paranoid and finds any little piece of hope that I am. She tells me this morning that when we get married she will be less paranoid. How would this make her less paranoid? Also, I dont think that I want to marry someone who constantly thinks I am cheating. I just dont know what to do. I really love and care about this woman. Any advice?