I can't cum during sex

It is very hard for me to cum while having regular sex. In fact I cannot remember ever having an orgasm that way. What seems to work though is to have a dildo in me (the vibrating kind) while my boyfriend performs oral sex with both is tongue and his fingers. It still seems to take a very long time, but it works. Also this very important - I have to have multiple partner thoughts of oral sex that I perform on them while I experience the pleasure. As soon as I begin to cum - my boyfriend penetrates me and that makes the whole orgasmic experience magnificent.
Do I tell my boy friend about my fantasy's - absolutely, it drives him crazy.