I can't relate to your experience, but I do have a little insight into this disorder, a long with other disorders..
I do understand that self inflicted pain is usually due to a person's inability to cope with pain or anger.. The only way to solve this in my mind is to get to the root of YOUR problem, because not all of us are alike.. Not all of us grew up like you, not all of us has dealt with what you deal with.. So I think you need to see a psychiatrist, and if you are 18 you are old enough to go to one without your parents knowing.. At least you can get to the root of this problem so you can present it to your parents without them getting defensive, because they are going to be the first to be looked at as root of this behavior.. I'm sure this is going to be something that you and your family are going to have to deal with and heal from.. But right now, you need to know why you do this, and how to heal from this.. Knowledge is Power.. So I suggest you start studying about why people cut.. Search for help online, programs for teens who cut..
While you are doing these things, search for God.. Decide if you are ready to be a Disciple of Christ, if you are ready to give your life to Him.. Surrender your weakness, surrender your life, pick up your Cross and Follow him.. You can't say to the Lord, I believe in you and be Saved.. You have to surrender your being, your soul, and all its earthly desires.. And He will begin to transform you into a beautiful image in likeness of Himself.. You will begin to radiate the beauty and glory of our Lord Jesus Christ.. What better way to heal yourself of this horrible torment than to listen to your God Given Conscience.. Your guide, The Holy Spirit!!
I would say, this is an opportunity to seek God, let him heal you, educate yourself, so that YOU can helps all God's children who are faced with the exact situation you are facing.. Because Sam, I don't know any people who are Self Mutilation Activists, and I know in this day in age.. We need people like you!!!
God Bless you Sam, and always know I'm here.. I have faith in you, and so does God..
My prayer to you!!

May the Blessings of God be upon you,
May His Peace abide in you,
May His presence Illuminate your heart,
Now and forever more, Amen....