I'm sixteen but I can't keep an erection up! Help!?

Hey, I'm a 16 year old dude and I'm having a problem. I get the normal no reason boners during the day and if I want to **** I can get it up too, but when i go to have sex he goes limp noodle. I tried looking at porn to get it up but nothing happened. Now, I've had sex with one girl before but it took a couple tries to get it up. But after that every other time was a breeze. Now I've tried with 4 other girls and it's a no go on all of them. What's wrong with me?!