Problem With Sex

I have the following problem I am 17 years old (boy) and until 3-4 months ago I masturbated regularly, but since then I have a girlfriend and I have sex with her regularly, but there is a problem ... when I'm in it after a while it starts to do i fall can anyone tell ladies what is the reason .. I personally read an article which says that if I masturbated too fast or if I was used to a narrower grip or something like that then it can not happen to me when I'm in it, because it is wider and it was a kind of erectile dysfunction ???