I have been there few times. The amount of incompetency will result to self doubt. It will kill you mentally. I suggest you talk to your boss and tell this story exactly as you described it here, if you are lucky, they will guide help you find your way, and if not, you will be fired and will do good for your mental health.

Once upon a time, when I was in that situation, I requested a salary cut for myself. (They promoted me instead).