You could look into short term disability and get the treatment you may need now- you may need to take a 'time out' and focus on yourself so you can get better in order to perform better at the job. Perhaps check with your HR department and check on their disability procedures. ((I've gone this route before)).

You mentioned that the enviornment you work is an "extremely unhealthy, unsupportive, stressful hell."
I believe ((through my own experience)) that these factors will contribute to your depression, confidence, and drain you of any positive energy. However I also know that looking for a NEW job when you are depressed seems extremely daunting...just a thought.

A couple of things I tried in my "toxic" work enviornment:

I tried to change the way I relate or think about my job and the people that were toxic to me. I know...this is easier said than done.

Can you listen to relaxing music while working to keep you calm?

Can you take a time out? Even if it is to go the restroom, lock the door and just be by yourself for 5 minutes?

I hope something works for you, I have BEEN there so beleive me when I say I know how you feel. good luck and know there are people here who care!