He cant get off...

My bf can't always get off... It makes me feel like something's wrong with me. He tells me it's not me and I'm sexy/gorgeous and he's just like that.. he can't always get off.. I've never had this problem with anyone in my past and idk what to think... And then I get kinda sad and try to not let it show but he can tell and. Then he gets sad that I'm sad. Can y'all please give me some sort of advice or something?..