My girlfriend and I have been going out for more than 3 years now, and when we try to have sex, I can't keep my erection for more than about 5 minutes. I know for a fact it's not her fault, because even when a masturbate, I lose my erection in about the same amount of time. I'm extremely worried, because I'm only 20 years old. I don't think it's anything mental, because I love her and am very attracted to her, so my only other explanation is that it is something physical; although I hope I'm wrong. Does anyone know what the problem maybe and how to fix it? I feel like this is ruining my life and any help would be greatly appreciated.