I am a Christian and she is not. We are in a relationship for thee months, and we had sex. I plan to live life fully with her. She even wants to believe in Christ after the preaching I did to her. I told her not to believe because of me. I said try searching for the true God yourself. After a few days she started praying and felt that God power really works. She prayed to the Lord Christ! What would you suggest I should do? I can't leave her, and she doesn't want to leave me. She says if her parents won't allow her to go with a Christian, she prefers to leave her parents and walk along with me! She also wants to be an disciple of Christ!
What you are trying to tell yourself is that if good can come from your sin, then the sin is justified. "And why not say, "Let us do evil that good may come"? -- as we are slanderously reported and as some affirm that we say. Their condemnation is just" (Romans 3:8).
No matter how you look at this situation you are sinning against the Lord you claim to be following. "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 are involved in fornication without any regret and somehow you think God is going to be pleased with you.
I'm glad your girlfriend is interested in Christ, but you have given her a false idea of what being a Christian is about. You are following the path that Peter warned against. "For when they speak great swelling words of emptiness, they allure through the lusts of the flesh, through lewdness, the ones who have actually escaped from those who live in error. While they promise them liberty, they themselves are slaves of corruption; for by whom a person is overcome, by him also he is brought into bondage" (II Peter 2:18-19). I don't know what you told her, but what you relayed to me is not what Christ or the apostles taught about becoming a Christian. It isn't about feelings, it is about submission to God. See: What Must I Do to be Saved?
I hope you both want to do what is right. The first thing is to repent of your sins, which means you must change your attitude toward sin -- no longer making excuses that it is acceptable -- and change your behavior. You can either marry your girlfriend or you live apart until you do get married. In regards to becoming a Christian, you both need to do fully what God requires of you to be saved from your sins.
"Judge not, that you be not judged. For with what judgment you judge, you will be judged; and with the measure you use, it will be measured back to you. And why do you look at the speck in your brother's eye, but do not consider the plank in your own eye? Or how can you say to your brother, 'Let me remove the speck from your eye'; and look, a plank is in your own eye? Hypocrite! First remove the plank from your own eye, and then you will see clearly to remove the speck from your brother's eye" (Matthew 7:1-5).
You are trying to lead a person to Christ while having a plank of sexual sin in your own eye. It is past time that you clean up your own life so you can help others clean up their own lives.