18 Year old with Erection Problem

Hi, I am only 18 but have great difficulty getting and maintaining a solid erection with my girlfriend. I just started taking an L-Arginine supplement but this just doesn't seem normal for an 18 year old. I'm not nervous and sometimes I can get one but maintaining it for more than a minute or two is an embarassing challenge. What can I do?

masturbate more, wake your penis up. spend a week (maybee 2) watching porn or going to see sum strippers & masturbating excessively. your penis will start reacting again. next time u see that girl, it'll work just fine.