I'm having a hard time getting my head round a few things. First, I was brought up in a loving Protestant Christian home and lost my way through high school. I now in my mid-twenties and I'm engaged to a Catholic. We've bought a house and are living together. We're due to get married in a few months. Things have happened that my relationship with God has came back. Now I don't know whether this is going to work. I love her very much and would like an opinion from someone who knows what God would want me to do.
What God wants anyone to do is to follow Him. "Let us hear the conclusion of the whole matter: Fear God and keep His commandments, for this is man's all. For God will bring every work into judgment, including every secret thing, whether good or evil" (Ecclesiastes 12:13-14).
By living together without being married, the two of you are engaged in fornication, which is sinful. "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). Until you get married, you should stop having sex and one should move out until after the wedding. If that is not practical, then you should go ahead and get married. You can always get married at the courthouse and then have your formal wedding later.
In regards to you marrying a Catholic, that is something you are going to have to make up your mind about. When two people of different beliefs marry, it becomes difficult when deciding what church to attend or what faith to raise your children in.
I'm glad you are finding your way back to God. I hope you find your way all the way back to Him. See What Must I Do to be Saved?