Ok, so it's really very wrong to equate getting an editorial letter to a world war, but on the most minute, warped, internal scale--especially to a debut author--it might not be that far off the mark.
You're going to open your inbox and be super excited that your awesome editor is actually writing to YOU! With even more awesome notes that is going to turn your humble little novel into a fantastic work of art! The world won't know how to handle the sheer level of your utter brilliance!
Of course, then you actually open said edit letter, and um, yeah, you might just panic. A little bit. Or, maybe, even a lot.
First off, this reaction is absolutely normal. You're being asked to make changes to something you've worked on for months, if not years. It's going to hurt, and it's more than likely going to be hard. At this first editorial pass, these changes might be major. We're talking theme, character overhauls, maybe getting rid of a character. Fitting in backstory, taking out backstory. Plot holes. Jiggling timelines. You know...pretty much your whole book (or so it feels like). Then there's still line edits and copy edits, the ones that deal with all the minor glitches like plot consistency, word choice, sentence structure--but this is a different kind of editing altogether, and comes later.
So have your moment (Now Panic and Freak Out)--eat a truckload of chocolate, have a good cry into your (hopefully very caffeinated) drink of choice--and then let it go.
Because you can do this.
You've already written an entire novel, one so good it made an actual publishing house give you money for the right to print it. That a real live editor wants to spend their time working with you. That in itself is an incredible accomplishment. So just think of these edits as tweaks, refining what's already there, like shaving off the fat to get to the good stuff. And when it's 3 am in the morning and you've only done half of what you meant to get done yesterday and you've got to get up in less than four hours...keep calm and carry on (meaning go to sleep already, it'll still be there in the morning).
Your editor already really, really loves your book. They only want to make it better, the best it can be. And with edit notes, they are giving you an opportunity to see your novel in a new light. How and where things can be improved, tightened, made absolutely amazing.
Your book's going to rock.