Sorry i'm not an expert but i really want to help you out based
on what i know.
Firstly i need to ask you that when you said that you are
masturbating for the past 20 years, do you also have ED? And when
did the symptoms appear?
Secondly, unhealthy eating habits and lifestyle like the intake
of drugs, alcohol and smoking can hamper your normal blood flow and
might affect your normal neurological functions which might cause
ED. Lack of excercise might also cause ED.
Thirdly, having low testosterone levels might affect your sex
drive and not only that but testosterone is also needed to keep
your penis erect.
PS: No offence but you might not be satisfied with your as she
might not be attractive or emotionally fulfilling. ( This point is
very important as this might be the cause of your ED)
Well, if you really are at wits end but you really want to marry
this girlfriend and engage in sexual activity with her. You could
try having foreplay before sex which might increase sexual arousal
of both parties. Who knows, you might not have ED when you engage
in sexual intercourse.
Sorry man, thats all i can think of. I guessed that what you
should really do is find another doctor in another hospital who
might be more experienced in dealing with the third case i have
mentioned. Or you could find a professional sex consultant who
might be able to help you too.
Thanks for reading bro, all the best for you.