Please Help. I am frigid, have a boyfriend and don't want to be frigid!?

I am a 17 year old girl, I have a boyfriend, and we've been going out for 4 months so far. and whenever he touches me i flinch, I trust him, and want to do stuff with him, but I get really scared i guess, I just want to run. i guess i am frigid, but i don't know what to do cause its not fair to him. Why do i feel this way, and What can i do about it?