You are probably thinking about it so much so when you feel like coming you get in trouble with yourself . Have you ever think about relaxation massages before sex ? it can really help you out a little bit especially because your partner is going to stimulate the blood that is running in your body and everybody knows our penis is full of blood ,it also give pleasure and your desire for sex is gonna go up wish makes easier to come > BUT if nothing like this works for you then you should talk to youer doctor just in case that something is not right.