I'm 50 and I can not keep my erect penis ...

Doctor, I'm 50 years old, I look like 35, I've been engaged for 11 years and she's 33 years old. only 4 years ago + - I do not have sex with her, I do not feel like it and I can not get an erection. Is it because I'm anxious, I work a lot at least 12 hours a day, will it be fatigue, stress? I can not keep my penis erect at the time of penetration, but when I'm alone I have a normal erection that stays upright almost 40 minutes, I wonder what I can do I'm afraid of losing my fiancée, she never complained about the situation, doctor, please orient me !
Expert answer:

You need to see a urologist.