Hello there -
ive had the same problem since I was your age ! Well actually at 19. Im now 28.
What you said is exactly what I experienced. Ive had all type of tests done, things u cant even imagine. Anyway, im sorry but there is no good news.
I was lucky enough to find a partner where sex wasnt everything. Anyway, we have broken up now and im starting to go insane.
Yes I have used viagra etc, but like yourself dont want to use it all the time.
What the doctors said to me was that I had something called venous leaks which is like a burst blood vessel in the penis. The only way of fixing it is to so call seal it by tying the veins together. Though the dr said it is a reoccuring thing which will also leave bumpy scars which I didnt want.
If u want any further info regarding tests etc, please feel free to email me !
Good luck