Most Helpful Guy
You just do. Gatta work through it as hard as it is. Just sit him down privately. Tell him you have something to tell him but it's very hard for you to talk about so be patient with you. Go from there. Why haven't you told your parents? They should probably know to?
Most Helpful Girl
It's not an easy thing to talk about. But you've been with him long enough to trust him I think. Just wait for an intimate moment, like when you're cuddling or lying in bed together, and say "there's something you should know about me."
Be prepared for him to freak out. I don't think he'll judge you, but he'll probably be very scared and sad that the girl he loves went through such a trauma. He may even react angrily, and want to punish whoever did that to you. Just reassure him and answer any questions he may have. You'll be fine. :)