scared of sex?

i've had sex before in my previous relationships, but for some reason in my current one i'm really nervous to have sex with him. we have literally gotten really close to having sex, but i always get really scared and we just stop everything. it's really frustrating for both of us because we both want to have sex and we've talked about it but something keeps stopping me. i've never had a bad experience with sex and i haven't been sexually abused so i don't know what could be causing this.