How to orgasm?

Ok so, I've had plenty of clitoral orgasms and they are amazing but, I've always wanted to orgasm through intercourse. I have never been able to, I'm not a slag but I've had a few sexual partners and I'm not settled down with my boyfriend who I'm so happy with. He keeps trying to make me orgasm through sex and for some reason I just can't! Sometimes he will go that deep I feel like it's in my stomach and I et scared rather than letting him Carry on I instantly without even realising pull away from him this happens in doggy.