I'm a big proponent of letting Rs and crits percolate for a few days before starting to work with them. It's never easy to learn the book isn't All That.
Yup. This. Of course, this applies more to a rigorous crit than a form rejection, which could mean everything or nothing.
I go through a broad spectrum of emotions after getting a rigorous critique. One of them is always the horrible, sinking feeling of
Ohhhh, no, I thought this was good; what's wrong with me? How can I possibly be a Real Writer if my self-critique skills are so poor? Etc., etc.
Once that sinking sensation has had time to dissipate, I realize the crit has taught me a lot. I still feel like an idiot, but I pick myself up, remember why I love my book, and make it better. (Of course, that depends on the book. Sometimes trunking it and moving on may feel like the best thing to do.)
I'm starting to believe that the only truly useful critiques come from readers who like
something about your manuscript
and are willing to tell you why it isn't fulfilling its potential. (Someone who just plain hates it, or didn't get past page 5, or "doesn't get it," won't have much to say that you can learn from. Neither will someone who praises every aspect of it to the skies.)
Finding such tough-yet-supportive readers for a particular ms. is extremely hard — busy agents don't have time for in-depth feedback unless they're considering representing you, and friends and family don't want to hurt the writer's feelings. I've had many readers who basically just stopped reading or told me, "It's confusing" — which let me know the book sucked, without giving me a path toward fixing it.
It's important to get that "meh" feedback, too, but it shouldn't be given undue weight. Sometimes particular concepts and genres just don't work for particular people. (A romance reader may not "get" space opera or vice versa.) If you bore or perplex someone who seems like the ideal target reader for your book,
that person is the one to seek detailed feedback from, because he or she may be able to explain exactly what's not working.