Anyone who's tried their hand at querying agents will tell you how nerve-racking and incredibly scary the process is. I hear of people who have been doing it for years--years--and I really have to admire their determination and perseverance. It can really take a lot out of you. Like your soul, if you're not careful enough.
The editing stage is blissful ignorance because it's just you and your book and you're calling all the shots, but then you get to writing your query letter and devouring Query Shark entries (pun not intended but appreciated) and it really hits you. "Oh crap, someone else decides if my book is good enough to publish. Someone else holds my entire writing career in the palm of their hand." *cue dramatic music*. And you have only a page's worth of space to convince that person that they're going to love your book (without actually coming right out and begging them to read the darned thing because, you promise, it'll be worth their time).
And, of course, there are always rejections. Hopefully these are