Most every man has that problem once in a while. When you're jerking off and when you're just about to cum, stop and let your dick get soft. Do this for several times before you actually cum. This helps you learn to control it better......Also, while you're fucking, try to think of something else such as what you did yesterday, etc. You could even think of your Mother-in Law!!
When you're fucking, and you feel like you're gonna cum, pull out and rub your dick on her clit until you gain control again, then put your dick back inside her. She'll like that and it helps you to last longer. That is what I call SLOW FUCKING. Most women really love it....The main thing is for you to focus on pleasing her and not yourself. Your tongue and fingers can still do wonders for her!!