How should you I tell my BF about past assault?

Hi ladies! I could use some help. I have been with my BF for 10 months. I was raped a few years ago. I want to tell him about it however I'm not sure how to bring it up. He is a great man and I don't want to scare him away. I haven't shared this in the past because the relationships were shorter and didn't feel ready. Any advice?