Thank you for sharing so honestly about what you have been walking through. Coming out of the new age and returning to Christ is a decisive turn, and it is understandable that you have felt intense opposition since then. The new age promises enlightenment and a new world, but it is empty and cannot save. Scripture is clear that salvation is through Jesus Christ alone, and when you come to Him you become a new creation. The old things pass away and all things become new, and God now claims you as His own son or daughter. He does not want a half hearted or lukewarm relationship, but a whole hearted loving relationship with you, not a legal or ritual one, but one based on faith in what Jesus has done. That new covenant stands and cannot fail because it rests on God's faithfulness, not yours, and it was sealed in the blood of Jesus for the forgiveness of sins.
Part of that new life is learning that your relationship with God is primary. When that relationship is right, it is meant to strengthen every other relationship. When it is not right, no other relationship can be right for long. That is why Scripture puts the commands about loving God first, because the heart of our problems is always our relationship with Him.
That is also why God's Word speaks so plainly about being unequally yoked together with unbelievers, asking what fellowship righteousness has with unrighteousness and what communion light has with darkness. That principle is not limited to marriage, but it certainly includes dating and marriage, and it is one of the most important human relationships you will ever enter. Many people believe their situation is the special exception that Scripture does not cover, telling themselves that love will change the other person or that God brought the relationship for that reason. Yet case after case shows the same painful outcome a year or two later, with regret and deep brokenness and the realization that ignoring the counsel of Scripture led to living in great distress. You are wise to take seriously that you are unequally yoked in some ways. Feelings of love, even genuine growing affection, and the fact that companionship eases loneliness, do not make an unequal yoke equal.
You also described that the spiritual attacks seemed to lessen after this relationship began, and to intensify when you consider ending it, and that the relief itself makes you want to stay. It is important not to use relief or pressure as your guide. The enemy can ease pressure to keep you in a compromise just as he can increase pressure when Christ is making strides in your life. A decrease in warfare is not proof that God has brought or approved a relationship, especially when that relationship does not align with Scripture. God's will is never revealed by a feeling or a circumstance that contradicts His Word. True peace and the fruit of love, joy, peace, patience, gentleness, goodness and self control come from being joined to Christ and learning to walk with Him, as one who walks with a close companion, not by using another person as shelter from battle.
The warfare you feel is real, but in Christ you are sealed and set apart. God makes a separation between those who are His and those who are not, and nothing can destroy you when you belong to Him. You are not expected to survive this in your own strength. The call is to walk after the Spirit and to live the new life you have been raised to in Christ, reckoning the old life dead and trusting His power day by day. That means not turning back to the weak and beggarly ways you left, and not clinging to a relationship for relief, but clinging to Christ Himself.
If this relationship is indeed an unequal yoke, the loving and right step is to step back from it romantically, even though that will be painful and lonely. God designed marriage to be a lifelong covenant of one man and one woman, and He designed sexual intimacy and living together to belong only within that covenant. Any relationship should be measured by that ethic, not by how it makes the warfare feel. Choosing obedience may not immediately lessen the battle, but it places you on solid ground where God can strengthen you, heal the loneliness in a healthy way, and lead you into fellowship where your faith is built up.
Praying for you now, that God would give you clear discernment through His Word, courage to obey it even when it costs you comfort, and deep assurance of His presence. Praying that He would guard you from deception, break every lingering hold of the new age, fill you with His Spirit, and help you to know His love more deeply so that you do not need to look to another person to survive the battle. Praying that He would surround you with wise and caring believers who can walk with you, that you would grow in daily prayer and Scripture, and that you would experience the victory and newness of life that comes from belonging fully to Jesus.