21yo with ED. Kind of Worried

Hey, Im 21. Ive had ED for about 2 years now. Its your typical ED story, or at least I think. I can get an errection just fine, but within a minute loose it, even if im in the middle of intercourse. I do smoke and have for the past 5years. When I was 18, I was in college "experimenting". I drank, smoked weed, and did occasional cocaine(once or twice a week). I didnt have the problem before I went to college, I didnt sleep around in college, so my thinking it would be the partying hard that I did. Since then, I have been drug free since I left my freshman year(at 18). I still smoke, and drink very rarely now that I work all the time. After reading on here, I have almost ruled out the possibility of an STD causing this. Is it true that cocaine use will cause ED, and if so....permanently? or would it be a higher chance of the smoking(Tobacco)(smoke a half pack or less a day) The doctor has given me Levitra, but at $80 for 4 pills, its getting a little pricy, plus, Id like to have my old self back.