Hi there Young man.
I note your peculiar problem with interest. Firstly, i do hope you are practicing safe sex & using a condom. Do remember you have 2 partners' & your first priority is to ensure all partners' are safe. I am a doctor & not a religious or preach so will refrain from preaching monogamy if the alternative seems to suit your needs & meets with the approval of all parties' concerned.
Remember women at different ages, have different requirements. Am assuming here, what you mean by 'more sex' is to last longer during a sexual encounter & /or more number of times. Is she keen on different types of sexual experiences? Perhaps you should try something different like 'role-playing' or perhaps you should focus more on the use of extensive foreplay & use your tongue & fingers to good effect!! It is very difficult to give you tips sans any inputs on the type of person she is..sometimes, it may be best if you talk to her & get her to guide you as to her particular requirements or needs. Am happy your girlfriend is not as demanding. Perhaps she is contended by nature or not been exposed to 'more' or 'better' sex & hence does not know the difference. Whatever be the case, wish you a good sex life & do be safe. Cheers!
Dr Praveen Rodrigues MD