Well, you've been patient with me and answered my questions during the penning of my first manuscript. Today, I sent it off. Instead of feeling delighted, as I did when the first draft came together, I feel something quite unexpected - depression. I miss my characters, I miss controlling their lives and I miss having a large work to fiddle around with, hone and perfect. It really is something close to grief, which I know is daft - I'm mourning for the figments of my imagination. Is this normal? Do lots of writers finish manuscripts and get the blues? What is the cure?
It's not at all daft to call it grief - that's just how human brains work. You were focused on something, quite intensely, and your brain got used to having it there. Now it's not there. That's how the grief mechanism works too (although the extent of grief is, no doubt, determined by how much time your brain has spent thinking about the person, place or novel). People grieve for all sorts of things and then tell themselves that it's silly, but it's not.
Writers tend to be passionate people; they're also isolated a lot of the time. Passion abhors a vacuum, and finalising a novel and sending it off produces a vacuum. So you need to fill the vacuum. Start creating your next story or, if that doesn't seem feasible, find another creative outlet. Sing, knit, cook, dance, whatever. But don't pathologise what you're feeling - it's normal, and you can make it better.
To answer your question about whether or not other writers get the blues when they finish something - I don't know. Whenever one of mine finishes something they have me standing over them pushing them to write something new. Well, actually, that's only true some of the time. Most of them just keep writing on their own. Often it's because they know they need to fend off the vacuum.
I could spend a lot of time talking about the 'creative personality' and what it needs, but I'll just sum it up by saying that once you've engaged in a massive creative task, like writing a novel, you need to keep that part of yourself active. If you do nothing, you're denying a huge part of your personality - and brain - what it needs. What you do is up to you - you don't necessarily have to write - but you do need to do something.