Why? What was it about your walk with God that allowed you to backslide?
God doesn't accept obedience to Him that isn't from a love motive. Do you love God? That's the reason you should desire to live a morally pure life. See Matthew 22:36-38 and 1 Corinthians 13:1-3.
True "castaways" don't desire to return to fellowship with God.
So, what does the Bible tell you is the means to victorious Christian living?
Only if you've rejected Christ's sacrifice in favour of a life of sin. So long as a person lives this way, the efficacy of Christ's sacrifice can have no benefit to them. Any time, though, that you want to return to fellowship with God, you may (1 John 1:9). And when you do, Christ's sacrifice is once more of benefit to you.
All of your doubts will dissolve when you love God with all of your being. Do you love Him above all? That's the key to resting secure in your relationship with God. When you do love Him above all, you will never wonder if you're one of His.
Romans 8:1
1 There is therefore now no condemnation for those who are in Christ Jesus.
How do you know you are in Christ Jesus?
1 John 2:3-5
3 Now by this we know that we know Him, if we keep His commandments.
4 He who says, "I know Him," and does not keep His commandments, is a liar, and the truth is not in him.
5 But whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him.
What is the First and Great commandment of God? Where does obedience to God begin?
Matthew 22:37-38
37 Jesus said to him, "'You shall love the Lord your God with all your heart, with all your soul, and with all your mind.'
38 This is the first and great commandment.
If you want to be confident that you are one of God's children, learn to love Him with all you are.
These thoughts won't bring you into loving fellowship with God. The apostle John explains:
1 John 4:16-19
16 And we have known and believed the love that God has for us. God is love, and he who abides in love abides in God, and God in him.
17 Love has been perfected among us in this: that we may have boldness in the day of judgment; because as He is, so are we in this world.
18 There is no fear in love; but perfect love casts out fear, because fear involves torment. But he who fears has not been made perfect in love.
19 We love Him because He first loved us.
Any thought or feeling that doesn't draw you nearer to God you ought to reject. Such thoughts and feelings are not of God. The devil, in particular, likes to drive a wedge of fear and condemnation between God and His children. Don't let him!
Let your mind dwell on the love that God has for you. Let it settle into your heart and mind. Believe it. And then daily, moment-by-moment, surrender yourself to the Holy Spirit who sheds the love of God abroad in our hearts. (Romans 5:5)