[Handcramp]

Cumming fast is really common, especially for young guys with raging hormones. It's not so much a problem if you are able to go again shortly after. One way to combat the issue is to practice edging. Masturbate to the point of being close to cumming, then back off before you cum. Try to maintain your erection and continue edging as long as you can. Part of what you are trying to do is to learn all of the subtle signals you feel from sexual stimulation which precede ejaculation and orgasm. Pay attention to those signals when having sex and you can learn to slow things a bit and hopefully prolong your session. You can always withdraw and give her oral or other stuff, too, as part of the sex. Consider sitting down with her and talking about it, if your relationship is serious enough. She probably would want to work with you to make things as good as possible.