Crying on the inside

I found out on Friday that a publisher has accepted my book proposal - I am converting my PhD thesis into a book. The completed manuscript is to be with them for the 1 March 2009. It will be tight but hopefully my teaching load at university won't be too bad. I also got another chapter accepted for publication in an edited volume. And a book arrived that I agreed to review for a journal. So it's all go. Yet, why do I feel so absolutely useless inside as if the publisher will realise that my work is shit and that it won't sell any copies (not that it will ever be a bestseller!). I am just so insecure about it all. Not helped by the fact that my former PhD supervisor's other PhD student is super confident and a real whiz kid with numerous publications already while I've always been the slower, dumber bumbling one. She got offered the first 2 jobs she interviewed for, it took me 50 applications before getting a job. Then I saw the list of the new people starting at the university with me and it included one girl who was unsuccessful but was clearly a reserve candidate as she is now on the list. I was told I was extremely high on the list of first choice candidates but I just feel now that they will suddenly realise that the other girl is much better than me and will get so much more help and support and go further than me.