Tao,
We have something in common, I start a new job tomorrow too. I don't think what you're going thru is irrational. it's what we feel. But these people wouldn't have hired us if they didn't think we could do the job, and we should feel good about that. You're going to be OK, and so am I.
Please post about this and let us know how your first day went. What kind of work are you doing? I't'll take awhile, but what you're feeling now will just be a distant memory. I'll be keeping you in my thoughts, and wishing you nothing but the best of luck!
Greg
"Beauty is truth, truth is beauty - that is all you know on earth, and all you need to know"