I can't cum while having sex!?

hi my boyfriend and i have had sex several times and have had oral sex multiple times as well, he can make me cum fast as hell when he eats me out but he never has made me cum while we've been having sex. we've tried plenty of different positions and nothing seems to work. I'm quite young so I'm not sure if that has anything to do with it? It bothers me tons though because I'm the only girl he's ever been with that he hasn't made cum which makes me feel shitty and that im not good at sex, etc!! Someone please help me figure out what the hell is going on!!