I used to have this problem really badly. Now my meds help a lot. I used to listen to relaxation/mindfulness meditation CDs on the way to work, at work at my desk, and then on the way home from work. Some nights I did mindfulness meditation when I got home too because I was so anxious/panicky. I started doing something from the Feel Good Handbook: draw a picture of yourself with a thought bubble and then write all your worries in there. For example, "I am afraid I will mess up today and then everyone will think I am a dummy and will laugh at me. I will never be able to do this job and will be a failure." Then I took a step back and realized that I had a half-decent reputation and if I messed up I could just dust off and carry on... so there was a process of examining and rethinking my thoughts that helped some.

I know how crippling the thoughts and fears can be. I wish you well.