losing ground

I've been fucking up enough at work that I've had serious fears of getting fired. It seems my mind has sunk into one of those states... and I just can't shake it. I'm bitter, defensive, and passive aggressive. I'm impatient, and I'm having trouble concentrating. I'm making careless mistakes. I've had to restrain myself from making flippant comebacks at the management. I rarely talk to anyone at work or elsewhere because my head always seems to be pointed down at the ground because I'm ashamed of who they'd see. I feel like I let them down because I haven't turned out to be as good at this job as they thought I could be. I feel like the whole world's over because if I can't do well at a job like this, how could I ever do anything better?

Will I always be such an emotional and mental wreck? Will I ever be close to normal? What can I do?