Reaching the Big O

Okay so I am 18, I've had four partners before and I still have not had an orgasm. Of course because we're young and we don't fully know what to do but I have a 24 year old boyfriend and his goal is to get me too since I told him I never have and he wants to help me feel that experience. We've tried but when I feel like I am close (isn't it like a feeling if you have to pee then you're close if you peed beforehand?) WELL, anyways.. I either stop him or I don't get close. Are they ways to hell me or am I stuck to this? I try not to stop him but I can't help it the feeling just kinda bothers me so I have trouble riding it out to finish. Any advice?