I can completely empathize with you. For me it was nerves. I would get so excited and would tense my cock to make it extra hard that I always came with just a couple pumps, and my orgasm was always coupled with "I'm sorry". Naturally it led to disinterest in sex with the girls I was with. I recall one time I was getting ready to fuck this girl and she was giving me head as I was unwrapping the condom and I didn't even last the amount of time that it took to unwrap it.
When I got together with my new girlfriend the first time, it was much the same and we'd been having these sexy conversations beforehand how I was going to fuck the shit out of her, then a minute later it was over.
I basically taught myself not to get so anxious, but what's more, taught myself not to apply so much pressure and tension on my cock. Basically you just need to relax. Now we fuck for one to two hours everytime, unless we choose to make it shorter and often enough I won't even cum at all if I don't want to. I also found that I have much greater control without a condom so I never use them anymore, but not all girls will let you get away with that! Very rarely do I cum without deciding to let myself cum, with one exception...which is the reason that I found this article.
When thunderfucking (i.e. very hard, very fast), I cannot go for more than a minute or two. Does anyone have suggestions for that?