I tend to think similarly, that it's probably not the greatest idea to tell your coworkers. Think about the cost benefit analysis of this. I mean, if you tell that one coworker about it, you might feel some temporary relief, but on the potential negative end, if that information that you shared gets around to those who you don't want knowing about your self-harm, then that will make you feel worse, I would think.

So you have on the potential costs end, something much worse than the potential benefits. Obviously this is my opinion and I'm not telling you to do one thing or another, but I would think about it before telling anyone.