First off, biting isn't weird. It involves both pain and concentration, so it really empties your head for a few seconds. It's how I keep myself from breaking things when I really get enraged. When you're depressed, you can see insult in things that weren't intended that way. The self-loathing tend to assume that others loathe them as well.

I don't know your situation. Maybe this is temporary, maybe you'll live with it your entire life. For both, every day is a struggle, for the latter it stays with you. I have a lot of experience with this, and I will certainly listen, if you need to talk. Talking to someone, expressing yourself, can make you feel surprisingly better, even at your worst. For those times when you can't express yourself, just keep yourself distracted. Sometimes, a good television show or song or game can save your life.