i never cum

my boyfriend and i have sex all the time and don't get me wrong..it's good sex! but i can never cum. he tries so hard and i feel bad because i think he feels like he's not doing a good job or i'm not enjoying, but i am. i get close sometimes but i never really cum/orgasm. when it's just me, a vibrator, and some porn, i come every time. but i never can with him. and it's not just him, i couldnt with the guy before him either (then again, that sex was terrible lol). but anyways idl why i can't but i really wish i could. anyone else have that problem? any tips?