I've known people who have had depression, some had severe depression. Really sucks so you have my sympathies. I also know there are no easy easy solutions unfortunately. Do seek therapy.
And, I know this much easier said than done, but do be sure to put your best effert in at work. You may think you're miserable now, but imagine how miserable you'd be if you were depressed *and*
I agree with those who suggest that it might be time to look around for a new job!