Can't cum??

So my I told my boyfriend before we were even going out that I could cum (but I couldn't) I just thought I would be weird if I said I couldn't so now when we have sex or we do oral I feel bad because I can't do it. Like I love it and it feels amazing and I feel like I'm going to orgasm or cum but I dont, either that or he just needs to go on for longer. I can orgasm myself but still can't cum. So is it that only some people can?