Difficulty Maintaining Erection.

I am 27 years of age, and have the habbit of watching pornography and masturbate from 7-8 years. Currently I am having difficulty maintaining erection without sexual thoughts and physical stimulation. With physical stimulation and no sexual thoughts I am able to get hard, but as soon as I stop stimulation and without sexual thoughts, within few seconds it starts going down. I noticed this by physically stimulating, and when getting completely hard I stop stimulation and starts the timer or starting counting. In 10-15 seconds it goes down. Is it normal or I have any physical problem like venous leakage. I read somewhere that average penis stay erect for 5-10 minutes without stimulation. I am worried about my situation.