WHY do i keep doing this to myself??

I'm a senior in high school with four weeks left in the school year. I've been picking since I was around 11, so about six years. I pick at my arms, my face, and sometimes when my anxiety gets really bad, my chest. Right now my face and my arms look basically disgusting, and what is even more sad than this is that before a week ago, I had gone a month without picking. I felt so good. I wore a strapless dress to prom, my skin was clear, I was eating healthy, I was happy, I was even exercising at the gym. But a week ago, something triggered my picking again, and I picked at my arms and at my face, and I've been picking at my face every day this week. I just don't understand how I can keep doing this, I know how terrible it makes me feel after I do it and I know that if I stop that my skin will clear up eventually, but for some reason I keep doing it. Its like almost not even a conscious thing. I usually pick when I'm stressed out, or when I'm really tired. These seem to be the triggers. The worst part is, I stop for weeks at a time and I'm doing great and I almost forget about it and then BAM , I wind up back in front of the mirror, or picking at the smallest little pores on my arms. What suprises me the most is that I can go from having close to clear skin to looking like I have horrible acne. And the worst part is I do it to myself. But I have made a promise to myself to NEVER pick at my skin again. I refuse to let this affect me any longer. Im going to try to keep up a blog on this website, and if anyone else wants to stop with me, cold turkey, once and for all, I encourage you to do it, and I will be struggling to stop and take control of my life, with you.