I also noticed how casually I took the news. Any feelings of doubt lasted as long as it took me to get through reading the letter. Nine months ago, it would have taken me maybe a day or two to get over a rejection and move on. But in the nine months since I sent out "Patient Zero", I've abandoned a big novel, stopped myself from wasting time on another dead-end novel, and turned over half a dozen story ideas in my head, most of which I decided simply weren't worth my time; they just didn't work for me.
As you write, you have to be your own content manager. By that, I mean you have to try to partition your mind into two aspects: the Writer and the Boss. The Writer does the grunt work, the research and the production of a story. The Boss assigns stories to the Writer; "Hey, Writer, what are you working on? Well, fuck that shit. I got an assignment for you."
It might sound weird, like something you'd expect from a guy with dissociative identity disorder, but it is something I encourage. When you act as the Boss, you endeavor to take a few steps back from a project and look at it objectively. You ask yourself, "Is this story really working out?" If it's not and you're looking at the project as a writer, you might have too much emotion invested in the work. You've spent time researching the material, and long hours have gone into writing it all up. You don't want to say that the time spent has been for nothing.
How do you bring out the Boss? You need time and distance. For example, if you finish a a draft for a novel, take a week or two off from it (but no more than a month). Give yourself time to forget some of the finer details and specifics of what you wrote. When you go back to proofread, first tell yourself to be objective; imagine that you're going over someone else's work rather than your own.