Here is my situation:
For the past month now I had been having sex with my girlfriend. We're both 18. I've always been a little religious, but coming from a un-religious background, I didn't have much knowledge; therefore, I had sex before being married, but I did not know that this was sin. This past week feels as if something is telling me to stop having sex because I am sinning against God. I've told my girlfriend and it really upset her. So what should I do? I don't want to sin against God, but I don't want to ruin my relationship. What do I do?
"He who loves father or mother more than Me is not worthy of Me. And he who loves son or daughter more than Me is not worthy of Me. And he who does not take his cross and follow after Me is not worthy of Me. He who finds his life will lose it, and he who loses his life for My sake will find it" (Matthew 10:37-39).
Right and wrong is not a matter of feelings but of facts. Feelings can serve as warning bells, but alone feelings can give false alarms or false encouragement. "He who trusts in his own heart is a fool, but whoever walks wisely will be delivered" (Proverbs 28:26). Just as we had fears of our parents' wrath when we did something wrong, we ought to fear God's wrath since He made us, He can destroy us. "And do not fear those who kill the body but cannot kill the soul. But rather fear Him who is able to destroy both soul and body in hell" (Matthew 10:28). Not that God is just waiting to destory you or I. God greatly loves us, but He also won't overlook sin. "Therefore consider the goodness and severity of God: on those who fell, severity; but toward you, goodness, if you continue in His goodness. Otherwise you also will be cut off" (Romans 11:28).
Rather than offering your girlfriend your feelings on the subject, point out what God has said about what you are doing. "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). "Fornication," by the way, means having sex without being married.
There can be a number of reasons why you deciding to no longer have sex with your girlfriend upsets her. I suspect that it is because she assumes that you stay with her because of the sex. Thus, she assumes that if the sex stops it is because you found someone else to have sex with. She might not really believe that you would actually stay with her because of who she is. If this is true, even in part, another thing you can tell her is that you realize that it isn't right to use her for your own pleasure and put her at risk of getting pregnant. You want sex to be special between the two of you and that you want to reserve it for marriage, when you have tied yourself to her. She may still not understand. After all, this is not the way other guys behave and it isn't what is promoted in movies and romance novels. " For we have spent enough of our past lifetime in doing the will of the Gentiles -- when we walked in lewdness, lusts, drunkenness, revelries, drinking parties, and abominable idolatries. In regard to these, they think it strange that you do not run with them in the same flood of dissipation, speaking evil of you" (I Peter 4:3-4). Offer her the best and understand that she has to decide if she will accept the best from you.
However things work out in this relationship, understand that when you follow God's teachings, things will improve in your life. You'll avoid pitfalls that make other guys' lives miserable. It is that you won't face hardships and disappointments, but that they will be fewer and you'll know how to deal with them.
Let me know when you are ready to give your life over to God. We'll have a lot to talk about.