Everywhere. Sometimes I just go on a rampage.

I've had this problem for a long time, since I was 13, and I'm 21 now. Some times my skin gets better but then, all of a sudden I just go on a mission and ruin it again. I always feel so guilty and stupid straight afterwards because I know it will take months to heal again. I tried going on the pill and that cleared by skin up but had other health complications so now I'm back with no medication. I want to be confident enough to wear singlet tops or a bikini but I just can't. I'm so self-conscious, particularly after I've just ruined one of my arms by picking at the sores there. I don't know what to do. I don't know what the underlying psychological problem is that's causing me to do this. I just need to stop and making myself promise to stop just isn't cutting it. It's moved to every part of my body, I'm just constantly checking over my entire body for something to touch or pick at or look at and it's becoming a massive problem because the problem used to only be confined to small areas. I'm really hurting - physically, emotionally, psychologically. I just can't deal with it anymore. I really need help. Does anyone else out there have this much stress about this issue, where they are looking for any opportunity to pick? It certainly feels like a 'ritual' to me. What are some tips to stop?