Me and my girlfriend..... HELP!!! Please?

Alrighty...... Me and my girlfriend have been dating for some time now. I took her verginaty. But im having a problem, we have done "it" 3 times now. But everytime i cant get hard..... Let alone bust.... What is this all about im only 16 has anyone else has this problem. (and for you info she is pretty good looking so that is deff no what it is. Please help me i getting depressed about this