I’ve gone through this type of anxiety about work. Sometimes I even got up and went to work early just to face it all and get stuff done. I wonder if you are able to talk to hour boss about your level of anxiety? Maybe you can’t, I don’t know. He or she may not be a reasonable person. You wouldn’t have to get overly personal if you did have a conversation. Just something like the level of stress from this client is taking it’s toll and something has got to give. Maybe something else can be taken off your plate so you can at least relax a little bit. I understand that not all work situations are conducive to straightforward communication though. I hope this gets better for you soon.