Guaranteed to make you feel better.
I cut myself & purge because it helps me deal with stress. I know it's a grave sin & I just started seeing a counselor because my confessor is kind of insisting on it, but I still don't really understand WHY it's ALWAYS a grave sin. How is that different from smoking/drinking alcohol in moderation? How is throwing up objectively worse than voluntarily inhaling smoke? If it was 20x a day I could see a problem, but if it's just to relieve stress a few times a week I don't see why it matters?
The basic principle, of course, is that we have to respect our bodies as temples of the Holy Spirit. We know that Jesus drank wine, of course, and we know that the human liver can naturally metabolize alcohol (in moderation; too much damages it). Sure, the human body can naturally heal itself from cutting, but at a much higher biological cost with higher risk of infection, etc. It also has physical manifestations that blur or blemish that "templehood" and your natural beauty.
Heavy alcohol use can sometimes be a silent cry for help or an escape. Cutting is practically always a silent cry for help or a misguided psychological form of escape.
So I would focus on that. If you recognize that stress is a problem in your life, then yes counseling is a very very good idea. You need to be able to deal with that stress in a healthy way; cutting and purging are definitely increasing the biological stress on your body, which then filters through to the mental/spiritual dimension too. So you see how this can become a vicious cycle, right?
Now for the technical theological part, but bear with me because it's not all that complicated. Cutting (and to a lesser extent purging) is considered "grave matter" in itself, since it's a form of bodily mutilation (obviously not the worst of those forms). In other words, it's something quite serious and can be a grave sin. However, for your action to really be a grave sin, you need full consciousness of what you're doing and full consent. "Full knowing" and "full wanting."
Normally in a case of self-mutilation, a person doesn't have full clarity and a totally free willpower. From the way you're discussing it, I imagine that it may well be the case with you, but that's really for your confessor to discern when you speak with him in Confession.
I do concur with his advice... it sounds to me like the way that God wants to give you peace is to start healing your desires, if that makes sense. That won't just come through prayer or through some sort of self-discipline if there are deeper root causes here -- which is probably the case -- that can best be addressed by a counselor.
Your counselor is going to be able to tell you after a little while where she/he thinks the root causes of it are for you, but let me just mention -- in case anyone else reading this is in a similar situation -- that self-image and "body image" problems are usually the cause of cutting and purging. In other words, though I may not even be able to put it into words perfectly, deep down I really don't like myself or don't consider myself attractive or lovable, and I want to be somebody else; so I end up cutting without really knowing why... it just helps me get my anger and shame resolved. Don't worry... God has better ways to heal that!
Hopefully your counselor can approach things from a Christian perspective, telling you the truth about yourself: you're a beloved son/daughter of God whom God loves so much that he wants to have you alongside him for all eternity in a glorified body that's so beautiful and so much a reflection of God's image that we can't even see them with our earthly eyes!
God bless you. Be beautiful.
- Father Shane