Answer: The thing to keep in mind is that even if you did not ejaculate until after an hour had passed, your girlfriend probably would not have an orgasm. She may not even enjoy vaginal stimulation no matter what you do. The majority of women require clitoral stimulation if they are to experience orgasm. If you are concerned about giving your girlfriend pleasure, orally and/or manually stimulate her vulva and clitoris to orgasm prior to intercourse. Make sure you take care of her needs before your own, as least on occasion.
Your ejaculating so soon should only be a problem if vaginal intercourse is the only type of sex you have with your partner. If sex is over after you have had your orgasm, then it is a problem. Consider having intercourse only on occasion, versus every time.
Tell your girlfriend what you told me concerning why you ejaculate so soon. I am sure she would like to know why, so she is not left guessing.
If she wants to experience more vaginal stimulation, use your fingers. Make sure your fingernails are trimmed and smooth, and that they and your hands are clean. Then, starting with one finger, caress and massage her vaginal walls. Do not just thrust in and out. When her vagina relaxes and opens, switch to two fingers, and so on. Doing this, will help relax her pelvic muscles so her vagina will not be so tight. Teach her how to do Kegels; search this website to learn more about these.
Buy some water-based lubricant like KY Jelly and use condom(s) to decrease the sensitivity of your penis during intercourse. You can also masturbate to orgasm together before you have intercourse, to help relieve some of the sexual excitement.
Finally, do not make the sole reason or purpose of sex, vaginal intercourse. On occasion, make her the center of attention. Focus on her sexual pleasure.