Afraid of sex

I'm afraid of sex. I have a great BF and he has only had sex with one person, his ex. I'm a virgin and he's perfectly okay with taking it slow, but I've have had discussions on the topic. Things got a little too real and I started crying. He felt horrible although I told him it wasn't his fault. I didn't vocalize how I was uncomfortable. I think I've realized I'm so afraid of sex because I'm afraid to grow up. I'm in high school and I still want to be a kid. I really like this guy and I like to kiss him and stuff but I have almost sexual remorse. Advice?