Why couldn't i *** during sex?

im a guy and me and my gf were having sex for the first time. we are both virgins and and deeply in love. we love each other a lot. for some reason i couldn't climax. it felt good but for some reason no matter how fast how deep how hard i was pounding her, she was screaming with massive orgasm and i didn't ***. btw shes smoking hot so its not that she doesn't have the body or face i get really horny during foreplay with her, but i couldn't ejaculate. it was at least 30 min. of intercourse with lots of foreplay at the beginning. i can easily get myself off in approximately 20 min. do i just have really high stamina or is there something wrong???