Nineteen year old male unable to maintain erection

Patient: I’m a 19 year old male and i believe i suffer from ED. The first time i had sex with my girlfriend (we were both 16) and once we had actually engaged in coitus i quickly lost my erection. Prior to this i could easily get and maintain an erection either during sexual contact with my girlfriend or when i masturbated. 3 years on i have the same problem – when we have sex it’s either rock solid or not fully hard – kind of 80% up. If its not rock hard it quickly becomes flaccid, often once we have started sex. It has got to the point where i try to avoid sex to avoid the embarrassment or try to have sex the moment i get an erection, in order to show my girlfriend i can do it… I feel awful and i know the problem upsets her even if she’d never say it. When we are in foreplay my mind is racing – will it get up, will it stay up, will she enjoy it, am i rubbish at sex… The problem has extended to maturation – i get frustrated that my penis seemingly cannot get fully hard. I worry if my girlfriend and i broke up it would put other women off me. What can i do?