I was with my girlfriend and we are going to have sex. We tried everything, but I just could not keep a erection long enough or it was not hard enough. I also did not wake up the following morning with an erection. What is going on with me? Why couldn't I keep an erection? Why is it not hard enough? This never happened to me! Please answer.
First off, let's talk about some things you don't want to hear. What you and your girlfriend did was wrong. Even though you couldn't complete the act, you were involved in fornication. "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God" (I Corinthians 6:9-10). You have no business putting your penis in a girl you haven't married. See: Why Sex Outside of Marriage is Wrong.
What happened is that you knew you were doing something wrong. You were nervous and worried about it. Such emotions are counter productive to maintaining an erection. Once the anxiety set in, it became self-perpetuating. I would not be surprised to hear that alcohol or some other drug was also involved, and that too would interfere with erections.
Rather than trying to sin and risk becoming a father at such a young age, you should be getting your life together and seeking to please God. Then when the proper time comes after marriage, you will find that sex is not only easy but joyful.