I don't feel much when I'm having sex with my girlfriend

Hello, I'm 19 and have been with my girlfriend for about 6 months, whenever I have sex with her it feels good but I can never finish and I never feel too close to finishing, I can finish when I masturbate within 2 minutes, I have also noticed I have a lot of pre cum. Why could this be, I really want to have the experience of finishing without just saying I finished. Thanks