I don't want to encourage SH, you should use this as an opportunity to get clean..
However, you've probably heard that thousands of times before, and considering I'm in the exact same situation right now, I shouldn't be one to tell you to get clean. I personally like to bruise or scratch myself, since both of those methods can easily be passed off as accidents. I'm a smoker, so occasionally i also burn myself with cigarettes, although this one would be harder to pass off as accident.
I hope you can get better soon, and you should really consider getting clean. I know it can be hard, but you should give it a try.