How long does your sex last?

I'm 18 and I've had a girl recently and we've been having sex a lot, my first time was october 4th of 2010 when I was 17, and I lasted a good.. 2 minutes.. I've had sex many more times since then and only last a little bit longer maybe a minute longer, and I try to please her instead of pounding away and not caring, should I just pound instead? I'm geting kind of embarrased cause she said it isn't to great cause I don't last that long, kind of lowerd my self esteem :(