21 years old and I am having erectile problems often, what can I do?

I am only 21 and I have trouble getting an erection all the time? I can take Ginkgo Biloba and it works great for a couple weeks then it seems like it does nothing.  I have been to the doctor about it and they said my testosterone levels were low but the doctor said that not low enough to cause any problems it was just near the bottom of the normal range. I also tried a Herb combination called "Testojack 100" which is a testosterone booster with ZMA, Tribulus and Long fellow Jack and it also worked great for about a month then it just seems like my body gets immuned and it does work anymore either. What can I do about this, it is starting cause me major problems in the bedroom?