Welcome back, it is good to hear form you again even though things do not sound that good.
I'm not going to give you a lecture about self harm, I'm sure you know at least some of the downsides and the fact other girls do it is no reason. It can be a method (not a good one) of coping with the hard stuff in life, but there are better ones wiht a lot less danger to you - and leave you feeling better. They can involve coping better and not letting things get to you so much.
Would you like ot say something abut the things that make you feel bad?
I know you do not want any adults involved, many people do not for a whole raft of reasons, sometimes quite good ones. I don't know enough about you situation to judge.
I do know that when I was in a very bad suicidal place I simply could not lift myself out of it. I did not have a clue how, and did not have the motivation either, it was all too hard. It did take outside help, and now my life is good. No urge to take my life, no realy bad episodes. I can enjoy things without worrying they will turn sour or be spoiled by others
Although you might hesitate can I suggest the Kids Help Line? They are sensible, not prone to overreact and panic, and pretty knowledgeable about young people and the problems they face - ones that are just as serious as those that are older face too. Sometimes the first time you make contact it may not seem to click with you, please keep trying until it does (it will:)
They have a web-chat service, though this tends to be a bit brief, but can be a good starting point. Then when you are ready you can speak on the phone -a better solution.
Rather than making the same mistake I did and going it alone, please make contact with someone who can help - and of course you know you are welcome here anytime.