I am 17 years old and i have a girlfriend but i can’t get a full hard

Patient: I am 17 years old and i have a girlfriend but i can’t get a full hard erection both when she is present or when she isn’t present. She is waiting to have sex for the first time while i have done it multiple times with no problems, why can i only get a half erection and not a full hard erection that will allow us to have sex? ps i am healthy and have no medical conditions

Doctor: