I used to self harm. I've been clean for about 2-3 years now with very rare occurrences few and far between. I still get the urge from time to time, its very daunting and overwhelming it can get so bad that I literally writhe with agony. I'm not sure exactly how to explain it other than it being some sort of overwhelming compulsion that caused me a lot of distress and anxiety. I don't recall any voices, but then again my memory can sometimes be fairly hazy though I doubt my episodes are exactly psychotic. I'm not exactly sure. The rubber band trick isn't always as useful when it comes to overwhelming urges. Maybe you can try to distract yourself from the urge somehow, such as listen to music. It can sometimes help to draw you attention away from the powerful urges. It doesn't always work to simply distract yourself though.

If I may ask, what do you use to harm yourself with? You can try to hide those things from yourself at some point, or have someone take them and put them somewhere where you can't get to them.

I hope I could help in some way.