Trouble getting off

I seem to have trouble with having the big O when my boyfriend and I have sex. I'll be super into it and have those mini climaxes, but nothing huge like when I masturbate. He's good about paying attention to pleasing me and he always gets me so so so close but I just always seem to lose it.

I think it may have something to do with me still being shy about sex? I've only been sexually active for a year and a half and I've been with my current boyfriend for about 5 months.

Does this sound like a mental block to you guys? Does anyone have tips for helping me get over it or for getting to the big O? I appreciate the help :)