cant achieve orgasm while having intercourse and losing erection during

Hello, i am a 15 year old male, I have been having a problem with being able to reach climax during intercourse, honestly i could care less but it seems to be giving my girlfriend confidence issues and i only want her to be happy, this makes that goal a bit difficult, i have tried everything from no masturbation for 2 weeks up to not using a condom, even without one it took 4 hours for me to even get close. also whenever i am in the middle of having sex i will lose my erection, i do not know why either, i do not believe it is nerves because i am completely comfortable with my girlfriend, i  have heard that energy drinks can make it difficult to get an erection but i very very rarely drink any, i also do not think it is erectile dysfunction because i can easily get an erection with my hand, even if i am sitting right in front of her... somebody plz help i do not want her to be sad anymore!!! btw srry for bad punctuation and grammar.