Premature ejaculation problems

I'm 18 years old and I've been having problems with premature ejaculation for about a year. and only 2 times i have lasted an extremely long time (and that happened to be with the same girl at 2 separate times in the year). why would that happen? because every other girl i ejaculate within seconds of entering her. I also was wondering what i could do to prevent/help this problem. Would drugs be a factor in whats causing this?