This is weird to try and answer, but do you masturbate? If you do, does it take you longer or fast to cum?
If you c*U*m fast when masturbating, then i suggest you change the way you have sex, maybe turn on the light if you dont have it on already, or dim it for effect, maybe don't have ablanket over you just a sheet or nothing, maybe before you have sex do more forplay and touching then while you have sex kiss more and ask her to touch you more while your doing it. Just find different ways you enjoy it and you might find a way that will make you go crazy, in a good way.
Lasting long isn't a bad thing though, some women would probably love their man to last that long so you should both be happy that you canlast that long so you can mess around more because sex should be fun for you both so you should enjoy how ever long it goes for.
Good luck.