I dont know what your girl friend is like, but I know when im being extra sexy
and you know, we're kissing and touching and I know he's gettin all hot and bothered, I'll be sure to give him a hand job or go down on him FIRST. Before he puts it inside me. Mean while you should also be touching her. Once you get that first one out of the way, you'll last so much longer. Also I completely agree with the others about concentrate Hard on something else COMPLETELY if you find yourself building up. Besides if you do come fist you can always use your cum to help you play with her clit till she comes..I guarentee you, you have a better chance of getting ANY girl to come from circling her clit than you do from thrusting anything inside her. If you can get that motion down, girls will care less how fast you come.
Anyway, like the last guy said, it'll get better in time.