It's hard to know how to counsel you, because I have no close friends or relations who have chosen to cut themselves. So you might be tempted to brush off my admonitions, because I don't know what you're going through.
It's true. I don't know what you're going through. But I have gone through difficult times, whether they're like yours or not.
My experience has taught me a few simple truths that have helped me conquer anger, fear, and doubt. God really exists, and he really cares. Not only that, he can give you the power to live a holy life, free from sin and totally given over to him and his kingdom.
I'm not sure what Scripture you have heard before about self-harm, but here are a couple, the latter of which is pretty forceful.
Or do you not know that your body is a temple of the Holy Spirit who is in you, whom you have from God, and that you are not your own? For you have been bought with a price: therefore glorify God in your body. (1 Corinthians 6:19-20)
Do you not know that you are a temple of God, and that the Spirit of God dwells in you? If any man destroys the temple of God, God will destroy him, for the temple of God is holy, and that is what you are. (1 Corinthians 3:16-17)
A Christian has the Spirit of God dwelling in her, so her body is holy and must not be tampered with. It is your gift to God to be used as a vessel of honor.
Because of these truths, I'm tempted to say, "Just stop it! Why in the world would you want to do something so awful and destructive to God's temple!?"
But would that help? Maybe for some people it would. I'm not sure. But sometimes, in order to be loving, we have to be blunt, so here goes. Your cutting is a selfish, sinful act that gives evidence that you lack true faith.
Blunt enough? I make no apologies because truth is truth, and I would never dare alter it. The Bible tells us that when we surrender to God, the old self is crucified along with its lusts and desires, and a new creation is formed. We are set free from sin, not just the guilt of sin, but sin itself.
To cement that in your mind, I suggest that you read Romans chapter six several times. That passage changed my life by convincing me that I really can live free from sin. Once you understand this truth, you won't suffer from the inward focus that drives you to punish yourself. Your entire focus will turn outward, toward God and others.
You see, all sin is rooted in selfishness, including cutting. When you die to self and God regenerates you, that inward focus fades away.
Here's what I suggest. Read Romans chapter six at least three times. Focus on verses 6, 11, and 18. Then, look at your skin in the place you most recently cut yourself. Lay your hand on that place and say, "God, I give you my body to you as your temple. Never again will I desecrate it, whether in inappropriate sexual contact, through substance abuse, or with a sharp-edged instrument. This flesh is crucified and given as a sacrifice to you."
After that, every time you look at that spot on your skin, you will be reminded of your promise. Remember that God gives you the power to resist every temptation. (1 Corinthians 10:13), and Jesus said, "If the son sets you free, you will be free indeed."
You can do this. Surrender all and be his temple. If you trust God to break your chains, the freedom you experience will be beyond all you can ask or imagine.
Feel free to write again. My wife and I will be praying for you.