hey tracy,
please please don't cancel! it is going to be hard but you can do it. what nigel described in his post is pretty much what my cbt therapist told me to do before my initial exposure challenges. if you are good at visualisation (which, sadly, i'm not) then it should really work.
i know you are so freaked out and worried about tomorrow - it's such a horrible feeling. and i know what you mean about knowing rationally that everything is fine, but not being able to physically accept it. i have a set of cards and at the top of each card i've written a negative statement that i'm in the habit of thinking about ('i'm rubbish at my job', 'no-one really likes me', 'i'm a total failure'.) then underneath i've written a whole list of reasons why the statement is a load of rubbish. whenever i'm feeling one of the negative statements i look at the relevant card.
anyway, this is just one of many ways you can challenge irrational thoughts - hope it helps!
good luck tomorrow, let us know how it goes,
henri x