There is no doubt that you are in a difficult and painful situation. Your question shows that you have not deafened your ears against God and His good will for you. Your concerns are the result of hearing God’s warning whispers of danger ahead. The Spirit is leading you into righteousness.
As God’s child, you have a responsibility to do what is right, regardless of how difficult it may be. Concentrate on what God has said in His Word, and trust Him to lead you in making your next decisions on the basis of His life principles and commands. Ceasing to cohabit with your fiancé or fiancée is the first step. Putting the marriage on hold is the next.
The Bible says that believers should only marry other believers (2 Corinthians 6:14; see also 1 Corinthians 7:39—this verse specifically applies to widows, but the principle of “he must belong to the Lord” is present). So, your engagement to an unbeliever is problematic.
As for your living arrangements, which you are convicted about, it is good to listen to your conscience. Romans 14:23 says, “If you have doubts about whether or not you should [do] something, you are sinning if you go ahead and do it. For you are not following your convictions. If you do anything you believe is not right, you are sinning” (NLT). You don’t feel right about it for a reason. God’s Word is clear on the matter of pre-marital sex: “It is God’s will that you should be sanctified: that you should avoid sexual immorality” (1 Thessalonians 4:3).
Your Christian testimony is at risk. Sexual sin is something God tells us to be very careful about: “Among you there must not be even a hint of sexual immorality, or of any kind of impurity, or of greed, because these are improper for God’s holy people” (Ephesians 5:3). Living together before marriage definitely “hints” at immorality.
God does not want you to live in a state of sin, guilty feelings, or doubt. To have His full peace, follow Him fully. While your happiness, fulfillment, and usefulness to God are important, something else is even more important, and it should be your primary consideration: the exaltation of God’s name and the true reflection of His likeness in you. These are the main reasons that He gave you repenting faith to receive Jesus Christ as your Savior and Lord. See 1 Corinthians 10:31–32 and Romans 8:28–30.
As a born-again child of God, you have a strong desire to do what is right and please the Lord in every way. The path to holiness is difficult and calls for radical change (see Matthew 5:29–30). But in the power of the Holy Spirit you can do it.
Will you trust God enough to do what He says? Do you trust that He really knows what is best for you? Do you believe God has your best interests at heart?
Satan is a deceiver. He would like to trick you into believing that, once you make a poor decision, you are forced to make another, or that the only way to relieve the problems of one bad decision is to make a worse one. But God provides grace to renounce the past, to gain freedom from Satan’s deceptions, to take back the ground given to Satan, and to be restored to the Savior’s fellowship. Read James 4:1–10, focusing on verses 7–8.
You can trust the Lord to lead you in the right way. His way is better than anything this world has to offer (Psalm 84:10).