Been down in the deep depression pit of darkness myself. Your career isn't worth losing your life.

There's lots of good advice in these comments. Don't make any final decisions in your current state of mind. Seriously. Seek help out of the darkness. Finding a therapist who is a good fit may take some doing.

For some great perspectives:

http://www.stevepavlina.com/

http://theancientwisdomproject.com/