I have thought and still sometimes think the same thing.
I also used to hurt myself the same way, hitting my head on either a concrete wall or my knuckles. (Sometimes scratching myself all over)
Thing is, it will all heal at some point out of nowhere. It happened with me. I thought it would never end and there were multiple times where I felt like "quitting" (Nearly did). But I kept going, and now I'm okay.
Just find something to live for.