Deep dark secret

I am a 19 year girl. I have been with my boyfriend for 7 months. I want to tell him my deep dark sercet. Only few people know about it. But I am not sure how to tell him or when. He is the type of guy who gets really mad when something bad happens to me. To the point where he will fight or anything to make sure I am safe. He is very protective over me. I am scared if I tell him the dark secret he will do something bad. My secret is very personal & depressing. Its really hard for me to talk about it. & it eats at me. But I really want to open up to him. Any help?