Erectile dysfunction. Of poor nerves.

I m 30 yr male suffering from erectile dysfunction for last 4 yrs. When i m in standing position then i never get erection. However if i m in sleeping position then with help of blow job i sometimes get partial erection but the erection does not get 100% hardness & during intercourse it get disappears within few seconds. During erection if i stand straight then my erection immediately get disappeared which makes me feel that my penis nerves are weak which can not hold weight of penis during erection. I takes viagra/vigora 100 mg occasionally 1-2 time in a month which help me to get erection (in sleeping position only) with hardness & my erection sustains for 1-2 minutes. However, even after taking above pill if i stand straight, my erection does not sustain as my nerves does not bear weight of penis. When i was in 20s & got natural erection, i used to try/pressurise pushing down the penis to hide/finish erection as early as possible which may be the main reason of poor nerves.