Erectile Dysfunction. What should i do?

I am suffering from Erectile dysfunction. 2 years back, I was having a really good time with my girlfriend and when we indulge in sex I lost my erection while we were changing our positions, I started feeling guilty and then she came forward and started giving me sympathy. I felt terrible. And then I used viagra the same night. I thought, one day it will be okay. There is nothing like ED. I used to masturbate a lot since 6 years. And after that incident, whenever I tried to masturbate, I used to lost my erection within seconds without even ejaculating. I think the blood doesn't retain in my penis. Am I suffering from venous leakage or is it because of the continuous masturbation, I am getting loose erection too. I tried to mix-up stress and masturbation because of some personal reasons. Is it the reason behind all this. Now a days, it's getting very hard for me to keep an erection. Due to all these reasons, I am getting depressed and can't get rid of headache. What should I do?