Erectile dyfunction at 19? hard to get erect...

I am a 19 year old man, who never lost his virginity. I believe in having one partner instead of many. This also leads me to masturbate where sometimes I'll do it 2 times a day, other times I'll just take a break. Recently I noticed that I can't get as hard as I used to. My penis may become erect for about a minute, however it limps if I don't keep a firm grip on it. I am seeing a girl, and its to the point when I first started messing around with her; fingering, eating out, etc I had an erection. Now if I do it, it is hard for me to become erect. I been considering taking Viagra, however at 19 I don't think I should. Comparing to last year, I was hard as a rock when the wind blew by. I do work out, have small stress in life, but nothing overwhelming. One thing is that I am kinda "harsh" when I masturbate. I like to grip hard. Would this qualify as erectile dyfunctionsy?
Are there cures like eating a lot of fruits or resting your testicles on ice or can this be a stress related?