Has anyone ever had the problem of not being able to cum?

I've had sex with a guy who's big down there (6in) and I'm turned on,but when he goes inside of me I don't really feel much. I do get a nice feeling if I finger myself,I guess that's my gspot, so I can't be numb right? But i don't understand why I can't cum :( I get so turned on sexually and when I try to release i can't :(