Re: erection issues at 18?
I would suggest that you, continue to be tested. At least every 6 months to a year. So often, doctors miss something, that's caught in another exam. I would also go to different urologist as well.
Something is wrong, and you shouldn't rest until you find out what it is. Are you a smoker?
Have your arteries been checked for plack? I've read that smoking clogs the penil arteries that circulate blood thru the penis. Are you diabetic? This could be a problem.