When you're a series writer, the two words you never want to see are The End. Although like any novelist you finish every series book you write, by the time you type the last paragraph of one story you're already thinking ahead and planning the next. Until you reach the finale, the last book in the series, and you know there won't be anymore. Then you spend a lot of time second-guessing yourself (Maybe I could do a Next Generation series?) or sulking (it's not fair. I never got the chance to write Book X, Y or Z.)
You may get the writer version of series separation anxiety. You ask yourself Was it really worth it? a couple thousand times. You probably spend a few hours digging through old boxes of series memorabilia: the letter from the Big Name who thought your first novel was dazzling and gave you The Blurb to End All Blurbs; the first glowing fan letter; the dried flower you saved from the big bouquet your spouse brought to your first booksigning, that non-hatchet review published in the glossy publishing trade (then you re-read the hatchet jobs that promised your series would tank by book three, and yeah, you glance at your ten-novel series and smirk a little.)
At some point during this resentful, teary-eyed self-pity fest, you know you have to begin the process that will allow you to let go and move on. Because if you don't you will never write anything else, or you'll quit Publishing, or you'll spend the rest of your years doing something else while trying to forget what was or wallowing in tragic seclusion over what might have been and blaming everyone but yourself for it.
I've ended enough novel series now that I feel like I should know every inch of this particular emotional rollercoaster. I ought to; I've already built and ridden it six times (it seems weirdly appropriate that StarDoc would be the seventh series I've ended or had to end.) I'm fortunate in that I have other, ongoing series to write and keep me productive, and a couple of new prospects that are starting to look pretty solid. It still hurts to let go, but it's the only way to move forward.
In the end, nothing should get between you and the writing. Not even the writing.